diff --git a/protobuf/CHANGELOG.md b/protobuf/CHANGELOG.md
index 1ec9d5f5..9d10a960 100644
--- a/protobuf/CHANGELOG.md
+++ b/protobuf/CHANGELOG.md
@@ -1,3 +1,12 @@
+## 4.2.0-wip
+
+* Internal refactoring to split the package into libraries. This allows
+  conditionally importing different libraries and improving performance by
+  using different encoding/decoding libraries based on the target platform.
+  ([#1026])
+
+[#1026]: https://github.com/google/protobuf.dart/pull/1026
+
 ## 4.1.1
 
 * Minimum SDK dependency bumped from 3.6.0 to 3.7.0. ([#1024])
diff --git a/protobuf/lib/protobuf.dart b/protobuf/lib/protobuf.dart
index b500e512..2c6ef839 100644
--- a/protobuf/lib/protobuf.dart
+++ b/protobuf/lib/protobuf.dart
@@ -7,61 +7,13 @@
 /// [1]: https://developers.google.com/protocol-buffers
 library;
 
-import 'dart:collection' show ListBase, MapBase;
-import 'dart:convert'
-    show
-        Utf8Decoder,
-        Utf8Encoder,
-        base64Decode,
-        base64Encode,
-        jsonDecode,
-        jsonEncode;
-import 'dart:math' as math;
-import 'dart:typed_data' show ByteData, Endian, Uint8List;
-
-import 'package:fixnum/fixnum.dart' show Int64;
-import 'package:meta/meta.dart' show UseResult;
-
-import 'src/protobuf/json_parsing_context.dart';
-import 'src/protobuf/permissive_compare.dart';
-import 'src/protobuf/type_registry.dart';
-
-export 'src/protobuf/type_registry.dart' show TypeRegistry;
-
-part 'src/protobuf/annotations.dart';
-part 'src/protobuf/builder_info.dart';
-part 'src/protobuf/coded_buffer.dart';
-part 'src/protobuf/coded_buffer_reader.dart';
-part 'src/protobuf/coded_buffer_writer.dart';
-part 'src/protobuf/consts.dart';
-part 'src/protobuf/exceptions.dart';
-part 'src/protobuf/extension.dart';
-part 'src/protobuf/extension_field_set.dart';
-part 'src/protobuf/extension_registry.dart';
-part 'src/protobuf/field_error.dart';
-part 'src/protobuf/field_info.dart';
-part 'src/protobuf/field_set.dart';
-part 'src/protobuf/field_type.dart';
-part 'src/protobuf/generated_message.dart';
-part 'src/protobuf/generated_service.dart';
-part 'src/protobuf/json.dart';
-part 'src/protobuf/message_set.dart';
-part 'src/protobuf/pb_list.dart';
-part 'src/protobuf/pb_map.dart';
-part 'src/protobuf/proto3_json.dart';
-part 'src/protobuf/protobuf_enum.dart';
-part 'src/protobuf/rpc_client.dart';
-part 'src/protobuf/unknown_field_set.dart';
-part 'src/protobuf/unpack.dart';
-part 'src/protobuf/utils.dart';
-part 'src/protobuf/wire_format.dart';
-
-// TODO(sra): Use `Int64.parse()` when available:
-// https://github.com/dart-lang/fixnum/issues/18.
-/// @nodoc
-Int64 parseLongInt(String text) {
-  if (text.startsWith('0x')) return Int64.parseHex(text.substring(2));
-  if (text.startsWith('+0x')) return Int64.parseHex(text.substring(3));
-  if (text.startsWith('-0x')) return -Int64.parseHex(text.substring(3));
-  return Int64.parseInt(text);
-}
+export 'src/protobuf/internal.dart'
+    hide
+        BuilderInfoInternalExtension,
+        ExtensionFieldSet,
+        ExtensionFieldSetInternalExtension,
+        FieldInfoInternalExtension,
+        FieldSet,
+        FieldSetInternalExtension,
+        GeneratedMessageInternalExtension,
+        MapFieldInfoInternalExtension;
diff --git a/protobuf/lib/src/protobuf/annotations.dart b/protobuf/lib/src/protobuf/annotations.dart
index 9ec997b6..fa274538 100644
--- a/protobuf/lib/src/protobuf/annotations.dart
+++ b/protobuf/lib/src/protobuf/annotations.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Annotation for marking accessors that belong together.
 class TagNumber {
diff --git a/protobuf/lib/src/protobuf/builder_info.dart b/protobuf/lib/src/protobuf/builder_info.dart
index 4ac8bc06..24af8110 100644
--- a/protobuf/lib/src/protobuf/builder_info.dart
+++ b/protobuf/lib/src/protobuf/builder_info.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Per-message type setup.
 class BuilderInfo {
@@ -28,7 +28,7 @@ class BuilderInfo {
   final Map<String, FieldInfo> byName = <String, FieldInfo>{};
 
   /// Mapping from `oneof` field [FieldInfo.tagNumber]s to the their indices in
-  /// [_FieldSet._oneofCases].
+  /// [FieldSet._oneofCases].
   final Map<int, int> oneofs = <int, int>{};
 
   /// Whether the message has extension fields.
@@ -487,3 +487,16 @@ class BuilderInfo {
         ?.call(rawValue);
   }
 }
+
+extension BuilderInfoInternalExtension on BuilderInfo {
+  GeneratedMessage makeEmptyMessage(
+    int tagNumber,
+    ExtensionRegistry? extensionRegistry,
+  ) => _makeEmptyMessage(tagNumber, extensionRegistry);
+
+  ProtobufEnum? decodeEnum(
+    int tagNumber,
+    ExtensionRegistry? registry,
+    int rawValue,
+  ) => _decodeEnum(tagNumber, registry, rawValue);
+}
diff --git a/protobuf/lib/src/protobuf/coded_buffer.dart b/protobuf/lib/src/protobuf/coded_buffer.dart
index 4166228c..92102654 100644
--- a/protobuf/lib/src/protobuf/coded_buffer.dart
+++ b/protobuf/lib/src/protobuf/coded_buffer.dart
@@ -2,9 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
-void _writeToCodedBufferWriter(_FieldSet fs, CodedBufferWriter out) {
+void _writeToCodedBufferWriter(FieldSet fs, CodedBufferWriter out) {
   // Sorting by tag number isn't required, but it sometimes enables
   // performance optimizations for the receiver. See:
   // https://developers.google.com/protocol-buffers/docs/encoding?hl=en#order
@@ -17,7 +17,7 @@ void _writeToCodedBufferWriter(_FieldSet fs, CodedBufferWriter out) {
 
   final extensions = fs._extensions;
   if (extensions != null) {
-    for (final tagNumber in _sorted(extensions._tagNumbers)) {
+    for (final tagNumber in sorted(extensions._tagNumbers)) {
       final fi = extensions._getInfoOrNull(tagNumber)!;
       out.writeField(tagNumber, fi.type, extensions._getFieldOrNull(fi));
     }
@@ -31,7 +31,7 @@ void _writeToCodedBufferWriter(_FieldSet fs, CodedBufferWriter out) {
 
 void _mergeFromCodedBufferReader(
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   CodedBufferReader input,
   ExtensionRegistry registry,
 ) {
@@ -404,7 +404,7 @@ void _mergeFromCodedBufferReader(
 void _readPackableToListEnum(
   List list,
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   CodedBufferReader input,
   int wireType,
   int tagNumber,
@@ -426,7 +426,7 @@ void _readPackableToListEnum(
 void _readRepeatedEnum(
   List list,
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   CodedBufferReader input,
   int tagNumber,
   ExtensionRegistry registry,
diff --git a/protobuf/lib/src/protobuf/coded_buffer_reader.dart b/protobuf/lib/src/protobuf/coded_buffer_reader.dart
index 2bb9c2ec..bb1f5855 100644
--- a/protobuf/lib/src/protobuf/coded_buffer_reader.dart
+++ b/protobuf/lib/src/protobuf/coded_buffer_reader.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Reader used for converting binary-encoded protobufs into
 /// [GeneratedMessage]s.
diff --git a/protobuf/lib/src/protobuf/coded_buffer_writer.dart b/protobuf/lib/src/protobuf/coded_buffer_writer.dart
index a8dffd80..312e8fdd 100644
--- a/protobuf/lib/src/protobuf/coded_buffer_writer.dart
+++ b/protobuf/lib/src/protobuf/coded_buffer_writer.dart
@@ -4,7 +4,7 @@
 
 // ignore_for_file: constant_identifier_names
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Writer used for converting [GeneratedMessage]s into binary
 /// representation.
diff --git a/protobuf/lib/src/protobuf/consts.dart b/protobuf/lib/src/protobuf/consts.dart
index 227188ac..22062b7d 100644
--- a/protobuf/lib/src/protobuf/consts.dart
+++ b/protobuf/lib/src/protobuf/consts.dart
@@ -2,16 +2,14 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
-
 /// Constant string value of `double.infinity.toString()` and the infinity
 /// value recognized by `double.parse(..)`.
-const _infinity = 'Infinity';
+const infinity = 'Infinity';
 
 /// Constant string value of `double.negativeInfinity.toString()` and the
 /// negative infinity value recognized by `double.parse(..)`.
-const _negativeInfinity = '-Infinity';
+const negativeInfinity = '-Infinity';
 
 /// Constant string value of `double.nan.toString()` and the NaN (not a number)
 /// value recognized by `double.parse(..)`.
-const _nan = 'NaN';
+const nan = 'NaN';
diff --git a/protobuf/lib/src/protobuf/exceptions.dart b/protobuf/lib/src/protobuf/exceptions.dart
index 3effb448..f55e9be0 100644
--- a/protobuf/lib/src/protobuf/exceptions.dart
+++ b/protobuf/lib/src/protobuf/exceptions.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 const _truncatedMessageText = '''
 While parsing a protocol message, the input ended unexpectedly
diff --git a/protobuf/lib/src/protobuf/extension.dart b/protobuf/lib/src/protobuf/extension.dart
index 27743b72..dc37b91f 100644
--- a/protobuf/lib/src/protobuf/extension.dart
+++ b/protobuf/lib/src/protobuf/extension.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// An object representing an extension field.
 class Extension<T> extends FieldInfo<T> {
diff --git a/protobuf/lib/src/protobuf/extension_field_set.dart b/protobuf/lib/src/protobuf/extension_field_set.dart
index 9e066435..64972003 100644
--- a/protobuf/lib/src/protobuf/extension_field_set.dart
+++ b/protobuf/lib/src/protobuf/extension_field_set.dart
@@ -2,15 +2,15 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
-class _ExtensionFieldSet {
-  final _FieldSet _parent;
+class ExtensionFieldSet {
+  final FieldSet _parent;
   final Map<int, Extension> _info = <int, Extension>{};
   final Map<int, dynamic> _values = <int, dynamic>{};
   bool _isReadOnly = false;
 
-  _ExtensionFieldSet(this._parent);
+  ExtensionFieldSet(this._parent);
 
   Extension? _getInfoOrNull(int tagNumber) => _info[tagNumber];
 
@@ -157,8 +157,8 @@ class _ExtensionFieldSet {
 
   bool get _hasValues => _values.isNotEmpty;
 
-  bool _equalValues(_ExtensionFieldSet? other) =>
-      other != null && _areMapsEqual(_values, other._values);
+  bool _equalValues(ExtensionFieldSet? other) =>
+      other != null && areMapsEqual(_values, other._values);
 
   void _clearValues() => _values.clear();
 
@@ -166,7 +166,7 @@ class _ExtensionFieldSet {
   ///
   /// Repeated fields are copied.
   /// Extensions cannot contain map fields.
-  void _shallowCopyValues(_ExtensionFieldSet original) {
+  void _shallowCopyValues(ExtensionFieldSet original) {
     for (final tagNumber in original._tagNumbers) {
       final extension = original._getInfoOrNull(tagNumber)!;
       _addInfoUnchecked(extension);
@@ -212,3 +212,9 @@ class _ExtensionFieldSet {
     }
   }
 }
+
+extension ExtensionFieldSetInternalExtension on ExtensionFieldSet {
+  Map<int, dynamic> get values => _values;
+  Iterable<int> get tagNumbers => _tagNumbers;
+  Extension? getInfoOrNull(int tagNumber) => _getInfoOrNull(tagNumber);
+}
diff --git a/protobuf/lib/src/protobuf/extension_registry.dart b/protobuf/lib/src/protobuf/extension_registry.dart
index bc2f8b46..2535b49c 100644
--- a/protobuf/lib/src/protobuf/extension_registry.dart
+++ b/protobuf/lib/src/protobuf/extension_registry.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// A collection of [Extension] objects, organized by the message type they
 /// extend.
diff --git a/protobuf/lib/src/protobuf/field_error.dart b/protobuf/lib/src/protobuf/field_error.dart
index 6c91a21b..c3dda123 100644
--- a/protobuf/lib/src/protobuf/field_error.dart
+++ b/protobuf/lib/src/protobuf/field_error.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Returns the error message for an invalid field value,
 /// or null if it's valid.
diff --git a/protobuf/lib/src/protobuf/field_info.dart b/protobuf/lib/src/protobuf/field_info.dart
index a30925e2..87d76c27 100644
--- a/protobuf/lib/src/protobuf/field_info.dart
+++ b/protobuf/lib/src/protobuf/field_info.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// An object representing a protobuf message field.
 class FieldInfo<T> {
@@ -60,7 +60,7 @@ class FieldInfo<T> {
   /// `tagNumber` of `result_per_page` field is 3.
   final int tagNumber;
 
-  /// Index of the field in [_FieldSet._values] list of this field's message.
+  /// Index of the field in [FieldSet._values] list of this field's message.
   ///
   /// The value is `null` for extension fields.
   final int? index;
@@ -148,7 +148,7 @@ class FieldInfo<T> {
        assert(!_isEnum(type) || valueOf != null);
 
   static MakeDefaultFunc? findMakeDefault(int type, dynamic defaultOrMaker) {
-    if (defaultOrMaker == null) return PbFieldType._defaultForType(type);
+    if (defaultOrMaker == null) return PbFieldType.defaultForType(type);
     if (defaultOrMaker is MakeDefaultFunc) return defaultOrMaker;
     return () => defaultOrMaker;
   }
@@ -238,8 +238,8 @@ class FieldInfo<T> {
   }
 
   /// Convenience method to thread this FieldInfo's reified type parameter to
-  /// `_FieldSet._ensureRepeatedField`.
-  PbList<T> _ensureRepeatedField(BuilderInfo meta, _FieldSet fs) {
+  /// `FieldSet._ensureRepeatedField`.
+  PbList<T> _ensureRepeatedField(BuilderInfo meta, FieldSet fs) {
     return fs._ensureRepeatedField<T>(meta, this);
   }
 
@@ -247,6 +247,11 @@ class FieldInfo<T> {
   String toString() => name;
 }
 
+extension FieldInfoInternalExtension<T> on FieldInfo<T> {
+  List<T> ensureRepeatedField(BuilderInfo meta, FieldSet fs) =>
+      _ensureRepeatedField(meta, fs);
+}
+
 final RegExp _upperCase = RegExp('[A-Z]');
 
 String _unCamelCase(String name) {
@@ -304,7 +309,7 @@ class MapFieldInfo<K, V> extends FieldInfo<PbMap<K, V>?> {
   FieldInfo get valueFieldInfo =>
       mapEntryBuilderInfo.fieldInfo[PbMap._valueFieldNumber]!;
 
-  PbMap<K, V> _ensureMapField(BuilderInfo meta, _FieldSet fs) {
+  PbMap<K, V> _ensureMapField(BuilderInfo meta, FieldSet fs) {
     return fs._ensureMapField<K, V>(meta, this);
   }
 
@@ -313,3 +318,8 @@ class MapFieldInfo<K, V> extends FieldInfo<PbMap<K, V>?> {
     return PbMap<K, V>(keyFieldType, valueFieldType);
   }
 }
+
+extension MapFieldInfoInternalExtension<K, V> on MapFieldInfo<K, V> {
+  Map<K, V> ensureMapField(BuilderInfo meta, FieldSet fs) =>
+      _ensureMapField(meta, fs);
+}
diff --git a/protobuf/lib/src/protobuf/field_set.dart b/protobuf/lib/src/protobuf/field_set.dart
index ec73665b..aee73b7b 100644
--- a/protobuf/lib/src/protobuf/field_set.dart
+++ b/protobuf/lib/src/protobuf/field_set.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 @pragma('vm:never-inline')
 @pragma('wasm:never-inline')
@@ -25,7 +25,7 @@ void _throwFrozenMessageModificationError(
 /// These fields and methods are in a separate class to avoid polymorphic
 /// access due to inheritance. This turns out to be faster when compiled to
 /// JavaScript.
-class _FieldSet {
+class FieldSet {
   final GeneratedMessage? _message;
 
   /// The value of each non-extension field in a fixed-length array.
@@ -34,7 +34,7 @@ class _FieldSet {
   final List _values;
 
   /// Contains all the extension fields, or null if there aren't any.
-  _ExtensionFieldSet? _extensions;
+  ExtensionFieldSet? _extensions;
 
   /// Contains all the unknown fields, or null if there aren't any.
   UnknownFieldSet? _unknownFields;
@@ -53,7 +53,7 @@ class _FieldSet {
   /// code as an `int`.
   Object _frozenState = false;
 
-  /// The [BuilderInfo] for the [GeneratedMessage] this [_FieldSet] belongs to.
+  /// The [BuilderInfo] for the [GeneratedMessage] this [FieldSet] belongs to.
   ///
   /// WARNING: Avoid calling this for any performance critical code, instead
   /// obtain the [BuilderInfo] on the call site.
@@ -82,7 +82,7 @@ class _FieldSet {
   /// the index is not present, the oneof field is unset.
   final Map<int, int>? _oneofCases;
 
-  _FieldSet(this._message, BuilderInfo meta)
+  FieldSet(this._message, BuilderInfo meta)
     : _values = _makeValueList(meta.byIndex.length),
       _oneofCases = meta.oneofs.isEmpty ? null : <int, int>{};
 
@@ -106,8 +106,8 @@ class _FieldSet {
   /// The [FieldInfo] for each non-extension field in tag order.
   Iterable<FieldInfo> get _infosSortedByTag => _meta.sortedByTag;
 
-  _ExtensionFieldSet _ensureExtensions() =>
-      _extensions ??= _ExtensionFieldSet(this);
+  ExtensionFieldSet _ensureExtensions() =>
+      _extensions ??= ExtensionFieldSet(this);
 
   UnknownFieldSet _ensureUnknownFields() {
     if (_unknownFields == null) {
@@ -510,7 +510,7 @@ class _FieldSet {
     _extensions?._clearValues();
   }
 
-  bool _equals(_FieldSet o) {
+  bool _equals(FieldSet o) {
     if (_meta != o._meta) return false;
     for (var i = 0; i < _values.length; i++) {
       if (!_equalFieldValues(_values[i], o._values[i])) return false;
@@ -545,7 +545,7 @@ class _FieldSet {
   }
 
   bool _equalFieldValues(Object? left, Object? right) {
-    if (left != null && right != null) return _deepEquals(left, right);
+    if (left != null && right != null) return deepEquals(left, right);
 
     final val = left ?? right;
 
@@ -584,7 +584,7 @@ class _FieldSet {
     }
 
     // Hash with descriptor.
-    var hash = _HashUtils._combine(0, _meta.hashCode);
+    var hash = HashUtils.combine(0, _meta.hashCode);
 
     // Hash with non-extension fields.
     final values = _values;
@@ -597,7 +597,7 @@ class _FieldSet {
     // Hash with extension fields.
     final extensions = _extensions;
     if (extensions != null) {
-      final sortedByTagNumbers = _sorted(extensions._tagNumbers);
+      final sortedByTagNumbers = sorted(extensions._tagNumbers);
       for (final tagNumber in sortedByTagNumbers) {
         final fi = extensions._getInfoOrNull(tagNumber)!;
         hash = _hashField(hash, fi, extensions._getFieldOrNull(fi));
@@ -605,7 +605,7 @@ class _FieldSet {
     }
 
     // Hash with unknown fields.
-    hash = _HashUtils._combine(hash, _unknownFields?.hashCode ?? 0);
+    hash = HashUtils.combine(hash, _unknownFields?.hashCode ?? 0);
 
     // Ignore _unknownJsonData to preserve existing hashing behavior.
 
@@ -625,18 +625,18 @@ class _FieldSet {
       return hash;
     }
 
-    hash = _HashUtils._combine(hash, fi.tagNumber);
+    hash = HashUtils.combine(hash, fi.tagNumber);
     if (_isBytes(fi.type)) {
       // Bytes are represented as a List<int> (Usually with byte-data).
       // We special case that to match our equality semantics.
-      hash = _HashUtils._combine(hash, _HashUtils._hashObjects(value));
+      hash = HashUtils.combine(hash, HashUtils.hashObjects(value));
     } else if (!_isEnum(fi.type)) {
-      hash = _HashUtils._combine(hash, value.hashCode);
+      hash = HashUtils.combine(hash, value.hashCode);
     } else if (fi.isRepeated) {
       final PbList list = value;
-      hash = _HashUtils._combine(
+      hash = HashUtils.combine(
         hash,
-        _HashUtils._hashObjects(
+        HashUtils.hashObjects(
           list.map((enm) {
             final ProtobufEnum enm_ = enm;
             return enm_.value;
@@ -645,7 +645,7 @@ class _FieldSet {
       );
     } else {
       final ProtobufEnum enm = value;
-      hash = _HashUtils._combine(hash, enm.value);
+      hash = HashUtils.combine(hash, enm.value);
     }
 
     return hash;
@@ -716,7 +716,7 @@ class _FieldSet {
   /// Singular fields that are set in [other] overwrite the corresponding fields
   /// in this message. Repeated fields are appended. Singular sub-messages are
   /// recursively merged.
-  void _mergeFromMessage(_FieldSet other) {
+  void _mergeFromMessage(FieldSet other) {
     // TODO(https://github.com/google/protobuf.dart/issues/60): Recognize
     // when `this` and [other] are the same protobuf (e.g. from cloning). In
     // this case, we can merge the non-extension fields without field lookups or
@@ -880,7 +880,7 @@ class _FieldSet {
   /// Makes a shallow copy of all values from [original] to this.
   ///
   /// Map fields and repeated fields are copied.
-  void _shallowCopyValues(_FieldSet original) {
+  void _shallowCopyValues(FieldSet original) {
     _values.setRange(0, original._values.length, original._values);
     final info = _meta;
     for (var index = 0; index < info.byIndex.length; index++) {
@@ -917,3 +917,26 @@ class _FieldSet {
     _oneofCases?.addAll(original._oneofCases!);
   }
 }
+
+extension FieldSetInternalExtension on FieldSet {
+  Iterable<FieldInfo> get infos => _infos;
+  Iterable<FieldInfo> get infosSortedByTag => _infosSortedByTag;
+  List get values => _values;
+  ExtensionFieldSet? get extensions => _extensions;
+  UnknownFieldSet? get unknownFields => _unknownFields;
+  Map<String, dynamic>? get unknownJsonData => _unknownJsonData;
+  set unknownJsonData(Map<String, dynamic>? value) => _unknownJsonData = value;
+  BuilderInfo get meta => _meta;
+  GeneratedMessage? get message => _message;
+  String get messageName => _messageName;
+
+  void ensureWritable() => _ensureWritable();
+  PbList<T> ensureRepeatedField<T>(BuilderInfo meta, FieldInfo<T> fi) =>
+      _ensureRepeatedField(meta, fi);
+  PbMap<K, V> ensureMapField<K, V>(BuilderInfo meta, MapFieldInfo<K, V> fi) =>
+      _ensureMapField(meta, fi);
+  void validateField(FieldInfo fi, dynamic newValue) =>
+      _validateField(fi, newValue);
+  void setFieldUnchecked(BuilderInfo meta, FieldInfo fi, dynamic value) =>
+      _setFieldUnchecked(meta, fi, value);
+}
diff --git a/protobuf/lib/src/protobuf/field_type.dart b/protobuf/lib/src/protobuf/field_type.dart
index cb6a1a47..ad3ac240 100644
--- a/protobuf/lib/src/protobuf/field_type.dart
+++ b/protobuf/lib/src/protobuf/field_type.dart
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // ignore_for_file: constant_identifier_names,non_constant_identifier_names
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 bool _isRepeated(int fieldType) => (fieldType & PbFieldType._REPEATED_BIT) != 0;
 
@@ -27,7 +27,7 @@ class PbFieldType {
   static int _baseType(int fieldType) =>
       fieldType & ~(_REQUIRED_BIT | _REPEATED_BIT | _PACKED_BIT | _MAP_BIT);
 
-  static MakeDefaultFunc? _defaultForType(int type) {
+  static MakeDefaultFunc? defaultForType(int type) {
     switch (type) {
       case _OPTIONAL_BOOL:
       case _REQUIRED_BOOL:
diff --git a/protobuf/lib/src/protobuf/generated_message.dart b/protobuf/lib/src/protobuf/generated_message.dart
index 7cf6a0a1..28f4992b 100644
--- a/protobuf/lib/src/protobuf/generated_message.dart
+++ b/protobuf/lib/src/protobuf/generated_message.dart
@@ -4,7 +4,7 @@
 
 // ignore_for_file: non_constant_identifier_names
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Type of an empty message builder.
 typedef CreateBuilderFunc = GeneratedMessage Function();
@@ -25,13 +25,13 @@ typedef ValueOfFunc = ProtobufEnum? Function(int value);
 /// `GeneratedMessage_reservedNames` and should be unlikely to be used in a
 /// proto file.
 abstract class GeneratedMessage {
-  _FieldSet? __fieldSet;
+  FieldSet? __fieldSet;
 
   @pragma('dart2js:tryInline')
-  _FieldSet get _fieldSet => __fieldSet!;
+  FieldSet get _fieldSet => __fieldSet!;
 
   GeneratedMessage() {
-    __fieldSet = _FieldSet(this, info_);
+    __fieldSet = FieldSet(this, info_);
 
     // The following two returns confuse dart2js into avoiding inlining the
     // constructor *body*. A `@pragma('dart2js:never-inline')` annotation on
@@ -637,3 +637,7 @@ extension GeneratedMessageGenericExtensions<T extends GeneratedMessage> on T {
   )
   T deepCopy() => info_.createEmptyInstance!() as T..mergeFromMessage(this);
 }
+
+extension GeneratedMessageInternalExtension on GeneratedMessage {
+  FieldSet get fieldSet => _fieldSet;
+}
diff --git a/protobuf/lib/src/protobuf/generated_service.dart b/protobuf/lib/src/protobuf/generated_service.dart
index f2b8857e..fc6feae3 100644
--- a/protobuf/lib/src/protobuf/generated_service.dart
+++ b/protobuf/lib/src/protobuf/generated_service.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Server side context.
 class ServerContext {
diff --git a/protobuf/lib/src/protobuf/internal.dart b/protobuf/lib/src/protobuf/internal.dart
new file mode 100644
index 00000000..176084d7
--- /dev/null
+++ b/protobuf/lib/src/protobuf/internal.dart
@@ -0,0 +1,67 @@
+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Runtime library for Dart implementation of [protobufs][1].
+///
+/// [1]: https://developers.google.com/protocol-buffers
+library;
+
+import 'dart:collection' show ListBase, MapBase;
+import 'dart:convert'
+    show
+        Utf8Decoder,
+        Utf8Encoder,
+        base64Decode,
+        base64Encode,
+        jsonDecode,
+        jsonEncode;
+import 'dart:math' as math;
+import 'dart:typed_data' show ByteData, Endian, Uint8List;
+
+import 'package:fixnum/fixnum.dart' show Int64;
+import 'package:meta/meta.dart' show UseResult;
+
+import 'consts.dart';
+import 'json_parsing_context.dart';
+import 'permissive_compare.dart';
+import 'type_registry.dart';
+import 'utils.dart';
+
+export 'type_registry.dart' show TypeRegistry;
+
+part 'annotations.dart';
+part 'builder_info.dart';
+part 'coded_buffer.dart';
+part 'coded_buffer_reader.dart';
+part 'coded_buffer_writer.dart';
+part 'exceptions.dart';
+part 'extension.dart';
+part 'extension_field_set.dart';
+part 'extension_registry.dart';
+part 'field_error.dart';
+part 'field_info.dart';
+part 'field_set.dart';
+part 'field_type.dart';
+part 'generated_message.dart';
+part 'generated_service.dart';
+part 'json.dart';
+part 'message_set.dart';
+part 'pb_list.dart';
+part 'pb_map.dart';
+part 'proto3_json.dart';
+part 'protobuf_enum.dart';
+part 'rpc_client.dart';
+part 'unknown_field_set.dart';
+part 'unpack.dart';
+part 'wire_format.dart';
+
+// TODO(sra): Use `Int64.parse()` when available:
+// https://github.com/dart-lang/fixnum/issues/18.
+/// @nodoc
+Int64 parseLongInt(String text) {
+  if (text.startsWith('0x')) return Int64.parseHex(text.substring(2));
+  if (text.startsWith('+0x')) return Int64.parseHex(text.substring(3));
+  if (text.startsWith('-0x')) return -Int64.parseHex(text.substring(3));
+  return Int64.parseInt(text);
+}
diff --git a/protobuf/lib/src/protobuf/json.dart b/protobuf/lib/src/protobuf/json.dart
index 75d4fa86..f2175675 100644
--- a/protobuf/lib/src/protobuf/json.dart
+++ b/protobuf/lib/src/protobuf/json.dart
@@ -2,9 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
-Map<String, dynamic> _writeToJsonMap(_FieldSet fs) {
+Map<String, dynamic> _writeToJsonMap(FieldSet fs) {
   dynamic convertToMap(dynamic fieldValue, int fieldType) {
     final baseType = PbFieldType._baseType(fieldType);
 
@@ -26,10 +26,10 @@ Map<String, dynamic> _writeToJsonMap(_FieldSet fs) {
       case PbFieldType._DOUBLE_BIT:
         final value = fieldValue as double;
         if (value.isNaN) {
-          return _nan;
+          return nan;
         }
         if (value.isInfinite) {
-          return value.isNegative ? _negativeInfinity : _infinity;
+          return value.isNegative ? negativeInfinity : infinity;
         }
         if (fieldValue.toInt() == fieldValue) {
           return fieldValue.toInt();
@@ -84,7 +84,7 @@ Map<String, dynamic> _writeToJsonMap(_FieldSet fs) {
   }
   final extensions = fs._extensions;
   if (extensions != null) {
-    for (final tagNumber in _sorted(extensions._tagNumbers)) {
+    for (final tagNumber in sorted(extensions._tagNumbers)) {
       final value = extensions._values[tagNumber];
       if (value is List && value.isEmpty) {
         continue; // It's repeated or an empty byte array.
@@ -105,7 +105,7 @@ Map<String, dynamic> _writeToJsonMap(_FieldSet fs) {
 // Merge fields from a previously decoded JSON object.
 // (Called recursively on nested messages.)
 void _mergeFromJsonMap(
-  _FieldSet fs,
+  FieldSet fs,
   Map<String, dynamic> json,
   ExtensionRegistry? registry,
 ) {
@@ -139,7 +139,7 @@ void _mergeFromJsonMap(
 
 void _appendJsonList(
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   List jsonList,
   FieldInfo fi,
   ExtensionRegistry? registry,
@@ -169,7 +169,7 @@ void _appendJsonList(
 
 void _appendJsonMap(
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   List jsonList,
   MapFieldInfo fi,
   ExtensionRegistry? registry,
@@ -178,7 +178,7 @@ void _appendJsonMap(
   final map = fi._ensureMapField(meta, fs);
   for (final jsonEntryDynamic in jsonList) {
     final jsonEntry = jsonEntryDynamic as Map<String, dynamic>;
-    final entryFieldSet = _FieldSet(null, entryMeta);
+    final entryFieldSet = FieldSet(null, entryMeta);
     final convertedKey = _convertJsonValue(
       entryMeta,
       entryFieldSet,
@@ -205,7 +205,7 @@ void _appendJsonMap(
 
 void _setJsonField(
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   json,
   FieldInfo fi,
   ExtensionRegistry? registry,
@@ -239,7 +239,7 @@ void _setJsonField(
 /// Throws [ArgumentError] if it cannot convert the value.
 dynamic _convertJsonValue(
   BuilderInfo meta,
-  _FieldSet fs,
+  FieldSet fs,
   value,
   int tagNumber,
   int fieldType,
diff --git a/protobuf/lib/src/protobuf/message_set.dart b/protobuf/lib/src/protobuf/message_set.dart
index 9a021317..dfae8770 100644
--- a/protobuf/lib/src/protobuf/message_set.dart
+++ b/protobuf/lib/src/protobuf/message_set.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 const _messageSetItemsTag = 1;
 const _messageSetItemTypeIdTag = 2;
diff --git a/protobuf/lib/src/protobuf/pb_list.dart b/protobuf/lib/src/protobuf/pb_list.dart
index 9d880f55..d48a8214 100644
--- a/protobuf/lib/src/protobuf/pb_list.dart
+++ b/protobuf/lib/src/protobuf/pb_list.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Type of a function that checks items added to a `PbList`.
 ///
@@ -204,10 +204,10 @@ class PbList<E> extends ListBase<E> {
 
   @override
   bool operator ==(Object other) =>
-      other is PbList && _areListsEqual(other, this);
+      other is PbList && areListsEqual(other, this);
 
   @override
-  int get hashCode => _HashUtils._hashObjects(_wrappedList);
+  int get hashCode => HashUtils.hashObjects(_wrappedList);
 
   void freeze() {
     if (_isReadOnly) {
diff --git a/protobuf/lib/src/protobuf/pb_map.dart b/protobuf/lib/src/protobuf/pb_map.dart
index 44337fb2..f9c5379d 100644
--- a/protobuf/lib/src/protobuf/pb_map.dart
+++ b/protobuf/lib/src/protobuf/pb_map.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// A [MapBase] implementation used for protobuf `map` fields.
 class PbMap<K, V> extends MapBase<K, V> {
@@ -77,7 +77,7 @@ class PbMap<K, V> extends MapBase<K, V> {
   int get hashCode {
     return _wrappedMap.entries.fold(
       0,
-      (h, entry) => h ^ _HashUtils._hash2(entry.key, entry.value),
+      (h, entry) => h ^ HashUtils.hash2(entry.key, entry.value),
     );
   }
 
@@ -108,7 +108,7 @@ class PbMap<K, V> extends MapBase<K, V> {
     final length = input.readInt32();
     final oldLimit = input._currentLimit;
     input._currentLimit = input._bufferPos + length;
-    final entryFieldSet = _FieldSet(null, mapEntryMeta);
+    final entryFieldSet = FieldSet(null, mapEntryMeta);
     _mergeFromCodedBufferReader(mapEntryMeta, entryFieldSet, input, registry);
     input.checkLastTagWas(0);
     input._currentLimit = oldLimit;
diff --git a/protobuf/lib/src/protobuf/proto3_json.dart b/protobuf/lib/src/protobuf/proto3_json.dart
index c2bbf29c..c1dbe11a 100644
--- a/protobuf/lib/src/protobuf/proto3_json.dart
+++ b/protobuf/lib/src/protobuf/proto3_json.dart
@@ -2,9 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
-Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
+Object? _writeToProto3Json(FieldSet fs, TypeRegistry typeRegistry) {
   String? convertToMapKey(dynamic key, int keyType) {
     final baseType = PbFieldType._baseType(keyType);
 
@@ -64,10 +64,10 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
         case PbFieldType._DOUBLE_BIT:
           final double value = fieldValue;
           if (value.isNaN) {
-            return _nan;
+            return nan;
           }
           if (value.isInfinite) {
-            return value.isNegative ? _negativeInfinity : _infinity;
+            return value.isNegative ? negativeInfinity : infinity;
           }
           if (value.toInt() == fieldValue) {
             return value.toInt();
@@ -160,7 +160,7 @@ extension _FindFirst<E> on Iterable<E> {
 /// to [fieldSet].
 void _mergeFromProto3Json(
   Object? json,
-  _FieldSet fieldSet,
+  FieldSet fieldSet,
   TypeRegistry typeRegistry,
   bool ignoreUnknownFields,
   bool supportNamesWithUnderscores,
@@ -173,7 +173,7 @@ void _mergeFromProto3Json(
     permissiveEnums,
   );
 
-  void recursionHelper(Object? json, _FieldSet fieldSet) {
+  void recursionHelper(Object? json, FieldSet fieldSet) {
     Object? convertProto3JsonValue(Object value, FieldInfo fieldInfo) {
       final fieldType = fieldInfo.type;
       switch (PbFieldType._baseType(fieldType)) {
diff --git a/protobuf/lib/src/protobuf/protobuf_enum.dart b/protobuf/lib/src/protobuf/protobuf_enum.dart
index 73779fb2..5ec61a0a 100644
--- a/protobuf/lib/src/protobuf/protobuf_enum.dart
+++ b/protobuf/lib/src/protobuf/protobuf_enum.dart
@@ -4,7 +4,7 @@
 
 // ignore_for_file: non_constant_identifier_names
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// A base class for all proto enum types.
 ///
diff --git a/protobuf/lib/src/protobuf/rpc_client.dart b/protobuf/lib/src/protobuf/rpc_client.dart
index 6239674e..7f091f08 100644
--- a/protobuf/lib/src/protobuf/rpc_client.dart
+++ b/protobuf/lib/src/protobuf/rpc_client.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Client side context for [RpcClient]s.
 class ClientContext {
diff --git a/protobuf/lib/src/protobuf/unknown_field_set.dart b/protobuf/lib/src/protobuf/unknown_field_set.dart
index b0579a91..62cee24b 100644
--- a/protobuf/lib/src/protobuf/unknown_field_set.dart
+++ b/protobuf/lib/src/protobuf/unknown_field_set.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// A set of unknown fields in a [GeneratedMessage].
 class UnknownFieldSet {
@@ -140,7 +140,7 @@ class UnknownFieldSet {
     if (other is! UnknownFieldSet) return false;
 
     final o = other;
-    return _areMapsEqual(o._fields, _fields);
+    return areMapsEqual(o._fields, _fields);
   }
 
   @override
@@ -159,7 +159,7 @@ class UnknownFieldSet {
   String _toString(String indent) {
     final stringBuffer = StringBuffer();
 
-    for (final tag in _sorted(_fields.keys)) {
+    for (final tag in sorted(_fields.keys)) {
       final field = _fields[tag]!;
       for (final value in field.values) {
         if (value is UnknownFieldSet) {
@@ -230,14 +230,14 @@ class UnknownFieldSetField {
     final o = other;
     if (lengthDelimited.length != o.lengthDelimited.length) return false;
     for (var i = 0; i < lengthDelimited.length; i++) {
-      if (!_areListsEqual(o.lengthDelimited[i], lengthDelimited[i])) {
+      if (!areListsEqual(o.lengthDelimited[i], lengthDelimited[i])) {
         return false;
       }
     }
-    if (!_areListsEqual(o.varints, varints)) return false;
-    if (!_areListsEqual(o.fixed32s, fixed32s)) return false;
-    if (!_areListsEqual(o.fixed64s, fixed64s)) return false;
-    if (!_areListsEqual(o.groups, groups)) return false;
+    if (!areListsEqual(o.varints, varints)) return false;
+    if (!areListsEqual(o.fixed32s, fixed32s)) return false;
+    if (!areListsEqual(o.fixed64s, fixed64s)) return false;
+    if (!areListsEqual(o.groups, groups)) return false;
 
     return true;
   }
diff --git a/protobuf/lib/src/protobuf/unpack.dart b/protobuf/lib/src/protobuf/unpack.dart
index aed3260b..08d54760 100644
--- a/protobuf/lib/src/protobuf/unpack.dart
+++ b/protobuf/lib/src/protobuf/unpack.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 /// Unpacks the message in [value] into [instance].
 ///
diff --git a/protobuf/lib/src/protobuf/utils.dart b/protobuf/lib/src/protobuf/utils.dart
index 4a6b0885..b4bb8bce 100644
--- a/protobuf/lib/src/protobuf/utils.dart
+++ b/protobuf/lib/src/protobuf/utils.dart
@@ -2,38 +2,38 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of '../../protobuf.dart';
+import 'internal.dart';
 
 // TODO(antonm): reconsider later if PbList should take care of equality.
-bool _deepEquals(Object lhs, Object rhs) {
+bool deepEquals(Object lhs, Object rhs) {
   // Some GeneratedMessages implement Map, so test this first.
   if (lhs is GeneratedMessage) return lhs == rhs;
   if (rhs is GeneratedMessage) return false;
-  if ((lhs is List) && (rhs is List)) return _areListsEqual(lhs, rhs);
-  if ((lhs is Map) && (rhs is Map)) return _areMapsEqual(lhs, rhs);
+  if ((lhs is List) && (rhs is List)) return areListsEqual(lhs, rhs);
+  if ((lhs is Map) && (rhs is Map)) return areMapsEqual(lhs, rhs);
   return lhs == rhs;
 }
 
-bool _areListsEqual(List lhs, List rhs) {
+bool areListsEqual(List lhs, List rhs) {
   if (lhs.length != rhs.length) return false;
   for (var i = 0; i < lhs.length; i++) {
-    if (!_deepEquals(lhs[i], rhs[i])) return false;
+    if (!deepEquals(lhs[i], rhs[i])) return false;
   }
   return true;
 }
 
-bool _areMapsEqual(Map lhs, Map rhs) {
+bool areMapsEqual(Map lhs, Map rhs) {
   if (lhs.length != rhs.length) return false;
-  return lhs.keys.every((key) => _deepEquals(lhs[key], rhs[key]));
+  return lhs.keys.every((key) => deepEquals(lhs[key], rhs[key]));
 }
 
-List<T> _sorted<T>(Iterable<T> list) => List.from(list)..sort();
+List<T> sorted<T>(Iterable<T> list) => List.from(list)..sort();
 
-class _HashUtils {
+class HashUtils {
   // Jenkins hash functions copied from
   // https://github.com/google/quiver-dart/blob/master/lib/src/core/hash.dart.
 
-  static int _combine(int hash, int value) {
+  static int combine(int hash, int value) {
     hash = 0x1fffffff & (hash + value);
     hash = 0x1fffffff & (hash + ((0x0007ffff & hash) << 10));
     return hash ^ (hash >> 6);
@@ -46,10 +46,10 @@ class _HashUtils {
   }
 
   /// Generates a hash code for multiple [objects].
-  static int _hashObjects(Iterable objects) =>
-      _finish(objects.fold(0, (h, i) => _combine(h, i.hashCode)));
+  static int hashObjects(Iterable objects) =>
+      _finish(objects.fold(0, (h, i) => combine(h, i.hashCode)));
 
   /// Generates a hash code for two objects.
-  static int _hash2(dynamic a, dynamic b) =>
-      _finish(_combine(_combine(0, a.hashCode), b.hashCode));
+  static int hash2(dynamic a, dynamic b) =>
+      _finish(combine(combine(0, a.hashCode), b.hashCode));
 }
diff --git a/protobuf/lib/src/protobuf/wire_format.dart b/protobuf/lib/src/protobuf/wire_format.dart
index a255c817..1dabb638 100644
--- a/protobuf/lib/src/protobuf/wire_format.dart
+++ b/protobuf/lib/src/protobuf/wire_format.dart
@@ -4,7 +4,7 @@
 
 // ignore_for_file: constant_identifier_names
 
-part of '../../protobuf.dart';
+part of 'internal.dart';
 
 const int _TAG_TYPE_BITS = 3;
 const int _TAG_TYPE_MASK = (1 << _TAG_TYPE_BITS) - 1;
diff --git a/protobuf/pubspec.yaml b/protobuf/pubspec.yaml
index 0b3a6103..50d99cf2 100644
--- a/protobuf/pubspec.yaml
+++ b/protobuf/pubspec.yaml
@@ -1,5 +1,5 @@
 name: protobuf
-version: 4.1.1
+version: 4.2.0-wip
 description: >-
   Runtime library for protocol buffers support. Use with package:protoc_plugin
   to generate Dart code for your '.proto' files.
diff --git a/protoc_plugin/test/reserved_names_test.dart b/protoc_plugin/test/reserved_names_test.dart
index 2158b9ab..56376c3f 100644
--- a/protoc_plugin/test/reserved_names_test.dart
+++ b/protoc_plugin/test/reserved_names_test.dart
@@ -20,7 +20,7 @@ void main() {
   test('GeneratedMessage reserved names are up to date', () {
     final actual = Set<String>.from(GeneratedMessage_reservedNames);
     final expected = findMemberNames(
-      'package:protobuf/protobuf.dart',
+      'package:protobuf/src/protobuf/internal.dart',
       #GeneratedMessage,
     );
 
@@ -30,7 +30,7 @@ void main() {
   test('ProtobufEnum reserved names are up to date', () {
     final actual = Set<String>.from(ProtobufEnum_reservedNames);
     final expected = findMemberNames(
-      'package:protobuf/protobuf.dart',
+      'package:protobuf/src/protobuf/internal.dart',
       #ProtobufEnum,
     );
 
@@ -39,7 +39,7 @@ void main() {
 
   test("ReadonlyMessageMixin doesn't add any reserved names", () {
     final mixinNames = findMemberNames(
-      'package:protobuf/protobuf.dart',
+      'package:protobuf/src/protobuf/internal.dart',
       #ReadonlyMessageMixin,
     );
     final reservedNames = Set<String>.from(GeneratedMessage_reservedNames);