Make DropDown accessible (#11149)

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

@@ -550,6 +550,12 @@ class RenderIndexedStack extends RenderStack {
     alignment: alignment
   );
 
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (index != null)
+      visitor(_childAtIndex());
+  }
+
   /// The index of the child to show, null if nothing is to be displayed.
   int get index => _index;
   int _index;

packages/flutter/test/material/dropdown_test.dart

@@ -8,6 +8,8 @@ import 'dart:ui' show window;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 
+import '../widgets/semantics_tester.dart';
+
 const List<String> menuItems = const <String>['one', 'two', 'three', 'four'];
 
 final Type dropdownButtonType = new DropdownButton<String>(
@@ -476,4 +478,16 @@ void main() {
     expect(find.byType(ListView, skipOffstage: false), findsNothing);
   });
 
+
+  testWidgets('Semantics Tree contains only selected element', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(buildFrame(items: menuItems));
+
+    expect(semantics, isNot(includesNodeWith(label: menuItems[0])));
+    expect(semantics, includesNodeWith(label: menuItems[1]));
+    expect(semantics, isNot(includesNodeWith(label: menuItems[2])));
+    expect(semantics, isNot(includesNodeWith(label: menuItems[3])));
+
+    semantics.dispose();
+  });
 }

packages/flutter/test/rendering/stack_test.dart

@@ -46,5 +46,35 @@ void main() {
     expect(green.size.height, equals(100.0));
   });
 
+
+  group('RenderIndexedStack', () {
+    test('visitChildrenForSemantics only visits displayed child', () {
+      final RenderBox child1 = new RenderConstrainedBox(
+          additionalConstraints: new BoxConstraints.tight(const Size(100.0, 100.0))
+      );
+      final RenderBox child2 = new RenderConstrainedBox(
+          additionalConstraints: new BoxConstraints.tight(const Size(100.0, 100.0))
+      );
+      final RenderBox child3 = new RenderConstrainedBox(
+          additionalConstraints: new BoxConstraints.tight(const Size(100.0, 100.0))
+      );
+      final RenderBox stack = new RenderIndexedStack(
+          children: <RenderBox>[child1, child2, child3],
+          index: 1,
+      );
+
+      final List<RenderObject> vistedChildren = <RenderObject>[];
+      final RenderObjectVisitor visitor = (RenderObject child) {
+        vistedChildren.add(child);
+      };
+
+      stack.visitChildrenForSemantics(visitor);
+
+      expect(vistedChildren, hasLength(1));
+      expect(vistedChildren.first, child2);
+    });
+
+  });
+
   // More tests in ../widgets/stack_test.dart
 }