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

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 781 matching lines @@
 abstract class App extends AbstractWidgetRoot {
 
   App({ RenderView renderViewOverride }) : super(renderViewOverride: renderViewOverride);
 
   void _buildIfDirty() {
     super._buildIfDirty();
 
     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();
 
 class RenderBoxToWidgetAdapter extends AbstractWidgetRoot {
 
   RenderBoxToWidgetAdapter(
     RenderObjectWithChildMixin<RenderBox> container,
     this.builder
   ) : _container = container, super() {
@@ skipping 24 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();
 }