Add button semantics to CupertinoButton (#12715)

fixes #11992 

packages/flutter/lib/src/cupertino/button.dart

@@ -195,6 +195,8 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
       onTapUp: enabled ? _handleTapUp : null,
       onTapCancel: enabled ? _handleTapCancel : null,
       onTap: widget.onPressed,
+      child: new Semantics(
+        button: true,
         child: new ConstrainedBox(
           constraints: widget.minSize == null
           ? const BoxConstraints()
@@ -234,6 +236,7 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
             ),
           ),
         ),
+      ),
     );
   }
 }

packages/flutter/test/cupertino/button_test.dart

@@ -2,10 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' show SemanticsFlags;
+
+import 'package:flutter/rendering.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../widgets/semantics_tester.dart';
+
 const TextStyle testStyle = const TextStyle(
   fontFamily: 'Ahem',
   fontSize: 10.0,
@@ -164,6 +169,37 @@ void main() {
     ));
     expect(opacity.opacity, pressedOpacity);
   });
+
+  testWidgets('Cupertino button is semantically a button', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(
+      boilerplate(
+          child: new Center(
+            child: new CupertinoButton(
+              onPressed: () { },
+              child: const Text('ABC')
+            ),
+          ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(
+      new TestSemantics.root(
+        children: <TestSemantics>[
+          new TestSemantics.rootChild(
+            actions: SemanticsAction.tap.index,
+            label: 'ABC',
+            flags: SemanticsFlags.isButton.index,
+          )
+        ],
+      ),
+      ignoreId: true,
+      ignoreRect: true,
+      ignoreTransform: true,
+    ));
+
+    semantics.dispose();
+  });
 }
 
 Widget boilerplate({ Widget child }) {