Exclude ModalBarrier from Semantics Tree if it is not dismissible (#10395)

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

@@ -2901,12 +2901,41 @@ class RenderMergeSemantics extends RenderProxyBox {
 
 /// Excludes this subtree from the semantic tree.
 ///
+/// When [excluding] is true, this render object (and its subtree) is excluded
+/// from the semantic tree.
+///
 /// Useful e.g. for hiding text that is redundant with other text next
 /// to it (e.g. text included only for the visual effect).
 class RenderExcludeSemantics extends RenderProxyBox {
   /// Creates a render object that ignores the semantics of its subtree.
-  RenderExcludeSemantics({ RenderBox child }) : super(child);
+  RenderExcludeSemantics({
+    RenderBox child,
+    bool excluding: true,
+  }) : _excluding = excluding, super(child) {
+    assert(_excluding != null);
+  }
+
+  /// Whether this render object is excluded from the semantic tree.
+  bool get excluding => _excluding;
+  bool _excluding;
+  set excluding(bool value) {
+    assert(value != null);
+    if (value == _excluding)
+      return;
+    _excluding = value;
+    markNeedsSemanticsUpdate();
+  }
 
   @override
-  void visitChildrenForSemantics(RenderObjectVisitor visitor) { }
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (excluding)
+      return;
+    super.visitChildrenForSemantics(visitor);
+  }
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('excluding: $excluding');
+  }
 }

packages/flutter/lib/src/widgets/basic.dart

@@ -3424,16 +3424,38 @@ class MergeSemantics extends SingleChildRenderObjectWidget {
 
 /// A widget that drops all the semantics of its descendants.
 ///
+/// When [excluding] is true, this widget (and its subtree) is excluded from
+/// the semantics tree.
+///
 /// This can be used to hide subwidgets that would otherwise be
 /// reported but that would only be confusing. For example, the
 /// material library's [Chip] widget hides the avatar since it is
 /// redundant with the chip label.
 class ExcludeSemantics extends SingleChildRenderObjectWidget {
   /// Creates a widget that drops all the semantics of its descendants.
-  const ExcludeSemantics({ Key key, Widget child }) : super(key: key, child: child);
+  const ExcludeSemantics({
+    Key key,
+    this.excluding: true,
+    Widget child,
+  }) : assert(excluding != null),
+        super(key: key, child: child);
+
+  /// Whether this widget is excluded in the semantics tree.
+  final bool excluding;
 
   @override
-  RenderExcludeSemantics createRenderObject(BuildContext context) => new RenderExcludeSemantics();
+  RenderExcludeSemantics createRenderObject(BuildContext context) => new RenderExcludeSemantics(excluding: excluding);
+
+  @override
+  void updateRenderObject(BuildContext context, RenderExcludeSemantics renderObject) {
+    renderObject.excluding = excluding;
+  }
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('excluding: $excluding');
+  }
 }
 
 /// A widget that builds its child.

packages/flutter/lib/src/widgets/modal_barrier.dart

@@ -26,7 +26,9 @@ class ModalBarrier extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return new Semantics(
+    return new ExcludeSemantics(
+      excluding: !dismissible,
+      child: new Semantics(
         container: true,
         child: new GestureDetector(
           onTapDown: (TapDownDetails details) {
@@ -43,6 +45,7 @@ class ModalBarrier extends StatelessWidget {
             )
           )
         )
+      )
     );
   }
 }

packages/flutter/test/widgets/modal_barrier_test.dart

@@ -4,8 +4,11 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
+import 'semantics_tester.dart';
+
 void main() {
   bool tapped;
   Widget tapTarget;
@@ -78,6 +81,40 @@ void main() {
     expect(find.byKey(const ValueKey<String>('barrier')), findsNothing,
       reason: 'The route should have been dismissed by tapping the barrier.');
   });
+
+  testWidgets('Undismissible ModalBarrier hidden in semantic tree', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(const ModalBarrier(dismissible: false));
+
+    final TestSemantics expectedSemantics = new TestSemantics.root();
+    expect(semantics, hasSemantics(expectedSemantics));
+
+    semantics.dispose();
+  });
+
+  testWidgets('Dismissible ModalBarrier includes button in semantic tree', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(const ModalBarrier(dismissible: true));
+
+    final TestSemantics expectedSemantics = new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          id: 1,
+          rect: TestSemantics.fullScreen,
+          children: <TestSemantics>[
+            new TestSemantics(
+              id: 2,
+              rect: TestSemantics.fullScreen,
+              actions: SemanticsAction.tap.index,
+            ),
+          ]
+        ),
+      ]
+    );
+    expect(semantics, hasSemantics(expectedSemantics));
+
+    semantics.dispose();
+  });
 }
 
 class FirstWidget extends StatelessWidget {