[web] Add 'flt-semantics-identifier' attribute to semantics nodes (#53278)

Make [`Semantics(identifier: '...')`](https://api.flutter.dev/flutter/semantics/SemanticsProperties/identifier.html) useful on the web. This PR plugs the Semantics `identifier` property as an HTML attribute `semantics-identifier` onto semantics elements.

This is useful in some scenarios:
- In testing to check if a certain semantics node has made it to the page ([example](flutter/flutter#97455)).
- In apps and/or packages to be able to lookup the DOM element that corresponds to a certain semantics node ([example](flutter/packages#6711)).

Fixes flutter/flutter#97455

Author: mdebbar

lib/web_ui/lib/src/engine/semantics/semantics.dart

@@ -612,6 +612,18 @@ abstract class PrimaryRoleManager {
     for (final RoleManager secondaryRole in secondaryRoles) {
       secondaryRole.update();
     }
+
+    if (semanticsObject.isIdentifierDirty) {
+      _updateIdentifier();
+    }
+  }
+
+  void _updateIdentifier() {
+    if (semanticsObject.hasIdentifier) {
+      setAttribute('flt-semantics-identifier', semanticsObject.identifier!);
+    } else {
+      removeAttribute('flt-semantics-identifier');
+    }
   }
 
   /// Whether this role manager was disposed of.
@@ -1119,6 +1131,21 @@ class SemanticsObject {
     _dirtyFields |= _headingLevelIndex;
   }
 
+  /// See [ui.SemanticsUpdateBuilder.updateNode].
+  String? get identifier => _identifier;
+  String? _identifier;
+
+  bool get hasIdentifier => _identifier != null && _identifier!.isNotEmpty;
+
+  static const int _identifierIndex = 1 << 25;
+
+  /// Whether the [identifier] field has been updated but has not been
+  /// applied to the DOM yet.
+  bool get isIdentifierDirty => _isDirty(_identifierIndex);
+  void _markIdentifierDirty() {
+    _dirtyFields |= _identifierIndex;
+  }
+
   /// A unique permanent identifier of the semantics node in the tree.
   final int id;
 
@@ -1278,6 +1305,11 @@ class SemanticsObject {
       _markFlagsDirty();
     }
 
+    if (_identifier != update.identifier) {
+      _identifier = update.identifier;
+      _markIdentifierDirty();
+    }
+
     if (_value != update.value) {
       _value = update.value;
       _markValueDirty();

lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -48,6 +48,9 @@ void runSemanticsTests() {
   group('longestIncreasingSubsequence', () {
     _testLongestIncreasingSubsequence();
   });
+  group(PrimaryRoleManager, () {
+    _testPrimaryRoleManager();
+  });
   group('Role managers', () {
     _testRoleManagerLifecycle();
   });
@@ -107,6 +110,68 @@ void runSemanticsTests() {
   });
 }
 
+void _testPrimaryRoleManager() {
+  test('Sets id and flt-semantics-identifier on the element', () {
+    semantics()
+      ..debugOverrideTimestampFunction(() => _testTime)
+      ..semanticsEnabled = true;
+
+    final SemanticsTester tester = SemanticsTester(owner());
+    tester.updateNode(
+      id: 0,
+      children: <SemanticsNodeUpdate>[
+        tester.updateNode(id: 372),
+        tester.updateNode(id: 599),
+      ],
+    );
+    tester.apply();
+
+    tester.expectSemantics('''
+<sem id="flt-semantic-node-0">
+  <sem-c>
+    <sem id="flt-semantic-node-372"></sem>
+    <sem id="flt-semantic-node-599"></sem>
+  </sem-c>
+</sem>''');
+
+    tester.updateNode(
+      id: 0,
+      children: <SemanticsNodeUpdate>[
+        tester.updateNode(id: 372, identifier: 'test-id-123'),
+        tester.updateNode(id: 599),
+      ],
+    );
+    tester.apply();
+
+    tester.expectSemantics('''
+<sem id="flt-semantic-node-0">
+  <sem-c>
+    <sem id="flt-semantic-node-372" flt-semantics-identifier="test-id-123"></sem>
+    <sem id="flt-semantic-node-599"></sem>
+  </sem-c>
+</sem>''');
+
+    tester.updateNode(
+      id: 0,
+      children: <SemanticsNodeUpdate>[
+        tester.updateNode(id: 372),
+        tester.updateNode(id: 599, identifier: 'test-id-211'),
+        tester.updateNode(id: 612, identifier: 'test-id-333'),
+      ],
+    );
+    tester.apply();
+
+    tester.expectSemantics('''
+<sem id="flt-semantic-node-0">
+  <sem-c>
+    <sem id="flt-semantic-node-372"></sem>
+    <sem id="flt-semantic-node-599" flt-semantics-identifier="test-id-211"></sem>
+    <sem id="flt-semantic-node-612" flt-semantics-identifier="test-id-333"></sem>
+  </sem-c>
+</sem>''');
+  });
+}
+
 void _testRoleManagerLifecycle() {
   test('Secondary role managers are added upon node initialization', () {
     semantics()