Re-wire up the back button

I broke this when I merged my AppContainer patch with Collin's back button
patch. This change restores the functionality I broke.

TBR=jackson@chromium.org

Review URL: https://codereview.chromium.org/1182463007.

sdk/lib/widgets/navigator.dart

@@ -73,6 +73,10 @@ class Navigator extends Component {
 
   NavigationState state;
 
+  void syncFields(Navigator source) {
+    state = source.state;
+  }
+
   void pushNamed(String name) {
     setState(() {
       state.pushNamed(name);

sdk/lib/widgets/widget.dart

@@ -813,6 +813,21 @@ class WidgetSkyBinding extends SkyBinding {
 }
 
 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() { }
 }