Re-wire up the back button

sky/sdk/lib/widgets/widget.dart

Index: sky/sdk/lib/widgets/widget.dart
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index e96f3d22db5626d9a5c5e2e8480c1eb3a85678ef..537537640a3e91c07a9c5683ec9c2f1208a698ec 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -771,6 +771,21 @@ class WidgetAppView extends AppView {
 }
 
 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() { }
 }