Refactor Navigator to put state in separate class, initial back button plumbing

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 7d450c98b11bdc330f7d7a1c48fb41004364fb4f..fcf6ebbceec18cfd02d9e82638ae873f67e9cc26 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -799,10 +799,17 @@ abstract class App extends AbstractWidgetRoot {
     if (root.parent == null) {
       // we haven't attached it yet
       WidgetAppView._appView.root = root;
+      WidgetAppView._appView.eventListeners.add((event) {

[Hixie, 2015/06/17 19:29:43]

This shouldn't use the underbar version. If there's no way to get to the appview
from here without using privates, then we have a bigger problem.

+        if (event.type == "back")
+          onBack();
+      });
     }
     assert(root.parent is RenderView);
   }
 
+  // Override this to handle back button behavior in your app
+  void onBack() { }
+
 }
 
 typedef Widget Builder();