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

sky/sdk/lib/app/view.dart

Index: sky/sdk/lib/app/view.dart
diff --git a/sky/sdk/lib/app/view.dart b/sky/sdk/lib/app/view.dart
index 4702ae4d03c04a69f69fe722f4095b41bfab9a38..b6022d15661d6bc87cd7b581cfc707ab3de55c70 100644
--- a/sky/sdk/lib/app/view.dart
+++ b/sky/sdk/lib/app/view.dart
@@ -50,6 +50,8 @@ class AppView {
 
   Function onFrame;
 
+  List<sky.EventListener> eventListeners = new List<sky.EventListener>();

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

Make this final, at least.

But ideally, we'd have an interface here where you can add (and remove)
listeners. Otherwise, random people can clear the list, send events to random
callbacks, etc.

Also, we shouldn't use sky.EventListener. Just use a typedef for a callback.

+
   RenderBox get root => _renderView.child;
   void set root(RenderBox value) {
     _renderView.child = value;
@@ -68,6 +70,10 @@ class AppView {
       HitTestResult result = new HitTestResult();
       _renderView.hitTest(result, position: new Point(event.x, event.y));
       dispatchEvent(event, result);
+    } else {
+      for (sky.EventListener listener in eventListeners) {
+        listener(event);
+      }
     }
   }