Back button should be able to close apps

sky/sdk/lib/widgets/widget.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
 
+import 'package:mojom/intents/intents.mojom.dart';
+import 'package:sky/mojo/shell.dart' as shell;
+
 import '../base/hit_test.dart';
 import '../rendering/box.dart';
 import '../rendering/object.dart';
 import '../rendering/sky_binding.dart';
 
 export '../rendering/box.dart' show BoxConstraints, BoxDecoration, Border, BorderSide, EdgeDims;
 export '../rendering/flex.dart' show FlexDirection;
 export '../rendering/object.dart' show Point, Offset, Size, Rect, Color, Paint, Path;
 
 final bool _shouldLogRenderDuration = false;
@@ skipping 924 matching lines @@
     super.didMount();
     SkyBinding.instance.addEventListener(_handleEvent);
   }
 
   void didUnmount() {
     super.didUnmount();
     SkyBinding.instance.removeEventListener(_handleEvent);
   }
 
   // Override this to handle back button behavior in your app
-  void onBack() { }
+  // Call super.onBack() to finish the activity
+  void onBack() {
+    ActivityManagerProxy activityManager = new ActivityManagerProxy.unbound();
+    shell.requestService(null, activityManager);

[abarth-chromium, 2015/07/07 00:14:24]

s/null/'mojo:sky_viewer'/

+    activityManager.ptr.finishCurrentActivity();
+  }
 }
 
 abstract class AbstractWidgetRoot extends Component {
 
   AbstractWidgetRoot() : super(stateful: true) {
     _mounted = true;
     _scheduleComponentForRender(this);
   }
 
   void syncFields(AbstractWidgetRoot source) {
@@ skipping 69 matching lines @@
     if (root.parent == null) {
       // we haven't attached it yet
       assert(_container.child == null);
       _container.child = root;
     }
     assert(root.parent == _container);
   }
 
   Widget build() => builder();
 }