Add button semantics to CupertinoButton (#12715)

fixes #11992 

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

@@ -195,39 +195,42 @@ class _CupertinoButtonState extends State<CupertinoButton> with SingleTickerProv
       onTapUp: enabled ? _handleTapUp : null,
       onTapCancel: enabled ? _handleTapCancel : null,
       onTap: widget.onPressed,
-      child: new ConstrainedBox(
-        constraints: widget.minSize == null
-            ? const BoxConstraints()
-            : new BoxConstraints(
-              minWidth: widget.minSize,
-              minHeight: widget.minSize,
-            ),
-        child: new FadeTransition(
-          opacity: _opacityTween.animate(new CurvedAnimation(
-            parent: _animationController,
-            curve: Curves.decelerate,
-          )),
-          child: new DecoratedBox(
-            decoration: new BoxDecoration(
-              borderRadius: widget.borderRadius,
-              color: backgroundColor != null && !enabled
-                  ? _kDisabledBackground
-                  : backgroundColor,
-            ),
-            child: new Padding(
-              padding: widget.padding ?? (backgroundColor != null
-                ? _kBackgroundButtonPadding
-                : _kButtonPadding),
-              child: new Center(
-                widthFactor: 1.0,
-                heightFactor: 1.0,
-                child: new DefaultTextStyle(
-                  style: backgroundColor != null
-                      ? _kBackgroundButtonTextStyle
-                      : enabled
-                          ? _kButtonTextStyle
-                          : _kDisabledButtonTextStyle,
-                  child: widget.child,
+      child: new Semantics(
+        button: true,
+        child: new ConstrainedBox(
+          constraints: widget.minSize == null
+          ? const BoxConstraints()
+          : new BoxConstraints(
+            minWidth: widget.minSize,
+            minHeight: widget.minSize,
+          ),
+          child: new FadeTransition(
+            opacity: _opacityTween.animate(new CurvedAnimation(
+                parent: _animationController,
+                curve: Curves.decelerate,
+            )),
+            child: new DecoratedBox(
+              decoration: new BoxDecoration(
+                borderRadius: widget.borderRadius,
+                color: backgroundColor != null && !enabled
+                ? _kDisabledBackground
+                : backgroundColor,
+              ),
+              child: new Padding(
+                padding: widget.padding ?? (backgroundColor != null
+                  ? _kBackgroundButtonPadding
+                  : _kButtonPadding),
+                child: new Center(
+                  widthFactor: 1.0,
+                  heightFactor: 1.0,
+                  child: new DefaultTextStyle(
+                    style: backgroundColor != null
+                    ? _kBackgroundButtonTextStyle
+                    : enabled
+                    ? _kButtonTextStyle
+                    : _kDisabledButtonTextStyle,
+                    child: widget.child,
+                  ),
                 ),
               ),
             ),

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 }) {
@@ -171,4 +207,4 @@ Widget boilerplate({ Widget child }) {
     textDirection: TextDirection.ltr,
     child: new Center(child: child),
   );
-}
\ No newline at end of file
+}