IconButton is a button, semantically (#13674)

* IconButton is a button, semantically

* fix datepicker test

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

@@ -191,7 +191,9 @@ class IconButton extends StatelessWidget {
     else
       currentColor = disabledColor ?? Theme.of(context).disabledColor;
 
-    Widget result = new ConstrainedBox(
+    Widget result = new Semantics(
+      button: true,
+      child: new ConstrainedBox(
         constraints: const BoxConstraints(minWidth: _kMinButtonSize, minHeight: _kMinButtonSize),
         child: new Padding(
           padding: padding,
@@ -210,6 +212,7 @@ class IconButton extends StatelessWidget {
             ),
           ),
         ),
+      ),
     );
 
     if (tooltip != null) {

packages/flutter/test/material/date_picker_test.dart

@@ -573,11 +573,13 @@ void _tests() {
             ],
           ),
           new TestSemantics(
+            flags: <SemanticsFlags>[SemanticsFlags.isButton],
             actions: <SemanticsAction>[SemanticsAction.tap],
             label: r'Previous month December 2015',
             textDirection: TextDirection.ltr,
           ),
           new TestSemantics(
+            flags: <SemanticsFlags>[SemanticsFlags.isButton],
             actions: <SemanticsAction>[SemanticsAction.tap],
             label: r'Next month February 2016',
             textDirection: TextDirection.ltr,

packages/flutter/test/material/icon_button_test.dart

@@ -2,10 +2,14 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui';
+
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../rendering/mock_canvas.dart';
+import '../widgets/semantics_tester.dart';
 
 class MockOnPressedFunction implements Function {
   int called = 0;
@@ -270,6 +274,32 @@ void main() {
 
     await gesture.up();
   });
+
+  testWidgets('IconButton Semantics', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      wrap(
+        child: new IconButton(
+          onPressed: mockOnPressedFunction,
+          icon: const Icon(Icons.link, semanticLabel: 'link'),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(new TestSemantics.root(
+      children: <TestSemantics>[
+        new TestSemantics.rootChild(
+          rect: new Rect.fromLTRB(0.0, 0.0, 48.0, 48.0),
+          actions: <SemanticsAction>[SemanticsAction.tap],
+          flags: <SemanticsFlags>[SemanticsFlags.isButton],
+          label: 'link',
+        )
+      ]
+    ), ignoreId: true, ignoreTransform: true));
+
+    semantics.dispose();
+  });
 }
 
 Widget wrap({ Widget child }) {