add label to drawer and regression test cases (#20537)

packages/flutter/lib/src/material/drawer.dart

@@ -401,8 +401,11 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
                   // On Android, the back button is used to dismiss a modal.
                   excludeFromSemantics: defaultTargetPlatform == TargetPlatform.android,
                   onTap: close,
+                  child: new Semantics(
+                    label: MaterialLocalizations.of(context)?.modalBarrierDismissLabel,
                     child: new Container(
-                    color: _color.evaluate(_controller)
+                      color: _color.evaluate(_controller),
+                    ),
                   ),
                 ),
               ),

packages/flutter/test/material/drawer_test.dart

@@ -2,9 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../widgets/semantics_tester.dart';
+
 void main() {
   testWidgets('Drawer control test', (WidgetTester tester) async {
     const Key containerKey = Key('container');
@@ -52,4 +56,54 @@ void main() {
 
     expect(find.text('header'), findsOneWidget);
   });
+
+  testWidgets('Drawer dismiss barrier has label on iOS', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Scaffold(
+          drawer: Drawer()
+        ),
+      ),
+    );
+
+    final ScaffoldState state = tester.firstState(find.byType(Scaffold));
+    state.openDrawer();
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(semantics, includesNodeWith(
+      label: const DefaultMaterialLocalizations().modalBarrierDismissLabel,
+      actions: <SemanticsAction>[SemanticsAction.tap],
+    ));
+
+    semantics.dispose();
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  testWidgets('Drawer dismiss barrier has no label on Android', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: const Scaffold(
+            drawer: Drawer()
+        ),
+      ),
+    );
+
+    final ScaffoldState state = tester.firstState(find.byType(Scaffold));
+    state.openDrawer();
+
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(semantics, isNot(includesNodeWith(
+      label: const DefaultMaterialLocalizations().modalBarrierDismissLabel,
+      actions: <SemanticsAction>[SemanticsAction.tap],
+    )));
+
+    semantics.dispose();
+  });
 }