Re-wire up the back button

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 '../app/view.dart';
@@ skipping 753 matching lines @@
         if (target is Listener)
           target._handleEvent(event);
         target = target._parent;
       }
     }
   }
 
 }
 
 abstract class App extends Component {
+  void _handleEvent(sky.Event event) {
+    if (event.type == 'back')
+      onBack();
+  }
+
+  void didMount() {
+    super.didMount();
+    WidgetAppView.appView.addEventListener(_handleEvent);
+  }
+
+  void didUnmount() {
+    super.didUnmount();
+    WidgetAppView.appView.removeEventListener(_handleEvent);
+  }
+
   // Override this to handle back button behavior in your app
   void onBack() { }
 }
 
 abstract class AbstractWidgetRoot extends Component {
 
   AbstractWidgetRoot() : super(stateful: true) {
     _mounted = true;
     _scheduleComponentForRender(this);
   }
@@ skipping 67 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();
 }