Replace json_utils with a modern extension type, add tests. (#52769)

I started on a refactoring of
flutter/flutter#147666, and was frustrated
with the JSON decoding of `gn desc --format=json`, so I used a similar
pattern from my micro-json library (https://pub.dev/packages/jsonut) and
encapsulated it as an extension type.

My favorite is the `JsonObject.map` function which replaces:

```dart
/// Construct a RunTarget from a JSON map.
factory RunTarget.fromJson(Map<String, Object> map) {
  final List<String> errors = <String>[];
  final String name = stringOfJson(map, _nameKey, errors)!;
  final String id = stringOfJson(map, _idKey, errors)!;
  final String targetPlatform =
      stringOfJson(map, _targetPlatformKey, errors)!;

  if (errors.isNotEmpty) {
    throw FormatException('Failed to parse RunTarget: ${errors.join('\n')}');
  }
  return RunTarget._(name, id, targetPlatform);
}
```

... with:

```dart
/// Construct a RunTarget from a JSON map.
factory RunTarget.fromJson(Map<String, Object> map) {
  return JsonObject(map).map((JsonObject json) => RunTarget._(
    json.string(_nameKey),
    json.string(_idKey),
    json.string(_targetPlatformKey),
  ));
}
```

Author: matanlurey

tools/engine_tool/lib/src/gn_utils.dart

@@ -11,8 +11,8 @@ import 'package:process_runner/process_runner.dart';
 
 import 'build_utils.dart';
 import 'environment.dart';
-import 'json_utils.dart';
 import 'proc_utils.dart';
+import 'typed_json.dart';
 
 /// Canonicalized build targets start with this prefix.
 const String buildTargetPrefix = '//';
@@ -86,9 +86,8 @@ Future<Map<String, BuildTarget>> findTargets(
       environment.logger
           .fatal('gn desc output is malformed $label has no value.');
     }
-    final Map<String, Object?> properties =
-        targetEntry.value! as Map<String, Object?>;
-    final String? typeString = getString(properties, 'type');
+    final JsonObject properties = JsonObject(targetEntry.value! as Map<String, Object?>);
+    final String? typeString = properties.stringOrNull('type');
     if (typeString == null) {
       environment.logger.fatal('gn desc is missing target type: $properties');
     }
@@ -97,7 +96,7 @@ Future<Map<String, BuildTarget>> findTargets(
       // Target is a type that we don't support.
       continue;
     }
-    final bool testOnly = getBool(properties, 'testonly');
+    final bool testOnly = properties.boolean('testonly');
     final List<String> outputs =
         await _runGnOutputs(buildDir.path, label, environment);
     File? executable;

tools/engine_tool/lib/src/json_utils.dart

@@ -3,16 +3,15 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 
 import 'package:path/path.dart' as p;
 import 'package:process_runner/process_runner.dart';
 
 import 'environment.dart';
-import 'json_utils.dart';
 import 'logger.dart';
+import 'typed_json.dart';
 import 'worker_pool.dart';
 
 /// Artifacts from an exited sub-process.
@@ -33,18 +32,18 @@ final class ProcessArtifacts {
 
   /// Constructs an instance of ProcessArtifacts from serialized JSON text.
   factory ProcessArtifacts.fromJson(String serialized) {
-    final Map<String, dynamic> artifact =
-        jsonDecode(serialized) as Map<String, dynamic>;
-    final List<String> errors = <String>[];
-    final Directory cwd = Directory(stringOfJson(artifact, 'cwd', errors)!);
-    final List<String> commandLine =
-        stringListOfJson(artifact, 'commandLine', errors)!;
-    final int exitCode = intOfJson(artifact, 'exitCode', errors)!;
-    final String stdout = stringOfJson(artifact, 'stdout', errors)!;
-    final String stderr = stringOfJson(artifact, 'stderr', errors)!;
-    final int? pid = intOfJson(artifact, 'pid', errors);
-    return ProcessArtifacts(cwd, commandLine, exitCode, stdout, stderr,
-        pid: pid);
+    final JsonObject artifact = JsonObject.parse(serialized);
+    return artifact.map((JsonObject json) => ProcessArtifacts(
+        Directory(json.string('cwd')),
+        json.stringList('commandLine'),
+        json.integer('exitCode'),
+        json.string('stdout'),
+        json.string('stderr'),
+        pid: json.integer('pid'),
+      ), onError: (JsonObject source, JsonMapException e) {
+        throw FormatException('Failed to parse ProcessArtifacts: $e', source.toPrettyString());
+      },
+    );
   }
 
   /// Constructs an instance of ProcessArtifacts from a file containing JSON.
@@ -54,16 +53,14 @@ final class ProcessArtifacts {
 
   /// Saves ProcessArtifacts into file.
   void save(File file) {
-    final Map<String, Object> data = <String, Object>{};
-    if (pid != null) {
-      data['pid'] = pid!;
-    }
-    data['exitCode'] = exitCode;
-    data['stdout'] = stdout;
-    data['stderr'] = stderr;
-    data['cwd'] = cwd.absolute.path;
-    data['commandLine'] = commandLine;
-    file.writeAsStringSync(jsonEncodePretty(data));
+    file.writeAsStringSync(JsonObject(<String, Object?>{
+      if (pid != null) 'pid': pid,
+      'exitCode': exitCode,
+      'stdout': stdout,
+      'stderr': stderr,
+      'cwd': cwd.absolute.path,
+      'commandLine': commandLine,
+    }).toPrettyString());
   }
 
   /// Creates a temporary file and saves the artifacts into it.

tools/engine_tool/lib/src/proc_utils.dart

@@ -7,7 +7,7 @@ import 'dart:convert';
 import 'package:process_runner/process_runner.dart';
 
 import 'environment.dart';
-import 'json_utils.dart';
+import 'typed_json.dart';
 
 const String _targetPlatformKey = 'targetPlatform';
 const String _nameKey = 'name';
@@ -17,16 +17,13 @@ const String _idKey = 'id';
 class RunTarget {
   /// Construct a RunTarget from a JSON map.
   factory RunTarget.fromJson(Map<String, Object> map) {
-    final List<String> errors = <String>[];
-    final String name = stringOfJson(map, _nameKey, errors)!;
-    final String id = stringOfJson(map, _idKey, errors)!;
-    final String targetPlatform =
-        stringOfJson(map, _targetPlatformKey, errors)!;
-
-    if (errors.isNotEmpty) {
-      throw FormatException('Failed to parse RunTarget: ${errors.join('\n')}');
-    }
-    return RunTarget._(name, id, targetPlatform);
+    return JsonObject(map).map((JsonObject json) => RunTarget._(
+      json.string(_nameKey),
+      json.string(_idKey),
+      json.string(_targetPlatformKey),
+    ), onError: (JsonObject source, JsonMapException e) {
+      throw FormatException('Failed to parse RunTarget: $e', source.toPrettyString());
+    });
   }
 
   RunTarget._(this.name, this.id, this.targetPlatform);