Revert "Framework sends a11y message when enabling semantics (#159163)" (#160039)

This reverts commit 1a2d6a3.

This is a straight revert as we are pivot away from using message
channel, instead we will be adding api to dart:ui.
See flutter/engine#56691

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above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
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Fixes] where supported.
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Author: chunhtai

packages/flutter/lib/src/rendering/binding.dart

@@ -790,20 +790,10 @@ class _BindingPipelineManifold extends ChangeNotifier implements PipelineManifol
       ChangeNotifier.maybeDispatchObjectCreation(this);
     }
     _binding.addSemanticsEnabledListener(notifyListeners);
-    if (_binding.semanticsEnabled) {
-      SystemChannels.accessibility.send(const GeneratingSemanticsTreeSemanticsEvent(true).toMap());
-    }
   }
 
   final RendererBinding _binding;
 
-  @protected
-  @override
-  void notifyListeners() {
-    SystemChannels.accessibility.send(GeneratingSemanticsTreeSemanticsEvent(_binding.semanticsEnabled).toMap());
-    super.notifyListeners();
-  }
-
   @override
   void requestVisualUpdate() {
     _binding.ensureVisualUpdate();

packages/flutter/lib/src/semantics/semantics_event.dart

@@ -135,40 +135,6 @@ class TooltipSemanticsEvent extends SemanticsEvent {
   }
 }
 
-/// An event to notify native OS that flutter starts or stops the semantics tree
-/// generation
-///
-/// The [generating] indicate whether flutter starts generating the semantics
-/// tree. If true, flutter will start sending semantics update to platform
-/// embedding.
-///
-/// Embeddings must be ready to receive semantics update after they receive this
-/// event with [generating] set to true as framework will start sending
-/// semantics update in the next frame.
-///
-/// If [generating] is false, embeddings need to clean up previous updates as
-/// the framework semantics tree was completely destroyed.
-class GeneratingSemanticsTreeSemanticsEvent extends SemanticsEvent {
-
-  /// Constructs an event that notify platform whether it is generating
-  /// semantics tree.
-  const GeneratingSemanticsTreeSemanticsEvent(this.generating)
-      : super('generatingSemanticsTree');
-
-  /// Whether framework starts generating the semantics tree.
-  ///
-  /// If true, flutter starts sending semantics update to platform
-  /// embedding.
-  final bool generating;
-
-  @override
-  Map<String, dynamic> getDataMap() {
-    return <String, dynamic> {
-      'generating': generating,
-    };
-  }
-}
-
 /// An event which triggers long press semantic feedback.
 ///
 /// Currently only honored on Android. Triggers a long-press specific sound

packages/flutter/test/rendering/binding_pipeline_manifold_semantics_event_test.dart

@@ -38,7 +38,7 @@ void main () {
 
     testWidgets('forTap', (WidgetTester tester) async {
       final SemanticsTester semanticsTester = SemanticsTester(tester);
-      semanticEvents.clear();
+
       await tester.pumpWidget(TestWidget(
         tapHandler: (BuildContext context) {
           return () => Feedback.forTap(context);
@@ -67,7 +67,7 @@ void main () {
 
     testWidgets('forTap Wrapper', (WidgetTester tester) async {
       final SemanticsTester semanticsTester = SemanticsTester(tester);
-      semanticEvents.clear();
+
       int callbackCount = 0;
       void callback() {
         callbackCount++;
@@ -102,7 +102,7 @@ void main () {
 
     testWidgets('forLongPress', (WidgetTester tester) async {
       final SemanticsTester semanticsTester = SemanticsTester(tester);
-      semanticEvents.clear();
+
       await tester.pumpWidget(TestWidget(
         longPressHandler: (BuildContext context) {
           return () => Feedback.forLongPress(context);
@@ -130,7 +130,6 @@ void main () {
 
     testWidgets('forLongPress Wrapper', (WidgetTester tester) async {
       final SemanticsTester semanticsTester = SemanticsTester(tester);
-      semanticEvents.clear();
       int callbackCount = 0;
       void callback() {
         callbackCount++;

packages/flutter/test/widgets/feedback_test.dart

@@ -280,7 +280,7 @@ void main() {
           'response': <String, dynamic>{},
         },
       );
-    }, semanticsEnabled: false); // Disabling semantics to prevent the unexpected message.
+    });
 
     testWidgets(
         'waiting for NoPendingPlatformMessages returns until a single method channel call returns', (WidgetTester tester) async {
@@ -313,7 +313,7 @@ void main() {
           'response': <String, dynamic>{},
         },
       );
-    }, semanticsEnabled: false); // Disabling semantics to prevent the unexpected message.
+    });
 
     testWidgets(
         'waiting for NoPendingPlatformMessages returns until both method channel calls return', (WidgetTester tester) async {
@@ -362,7 +362,7 @@ void main() {
           'response': <String, dynamic>{},
         },
       );
-    }, semanticsEnabled: false); // Disabling semantics to prevent the unexpected message.
+    });
 
     testWidgets(
         'waiting for NoPendingPlatformMessages returns until new method channel call returns', (WidgetTester tester) async {
@@ -413,7 +413,7 @@ void main() {
           'response': <String, dynamic>{},
         },
       );
-    }, semanticsEnabled: false); // Disabling semantics to prevent the unexpected message.
+    });
 
     testWidgets(
         'waiting for NoPendingPlatformMessages returns until both old and new method channel calls return', (WidgetTester tester) async {
@@ -463,7 +463,7 @@ void main() {
           'response': <String, dynamic>{},
         },
       );
-    }, semanticsEnabled: false); // Disabling semantics to prevent the unexpected message.
+    });
   });
 
   group('getSemanticsId', () {