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

sky/examples/stocks2/lib/stock_app.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 'package:sky/rendering/box.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/widget.dart';
 
 import 'stock_home.dart';
 import 'stock_settings.dart';
 
 class StocksApp extends App {
 
   StocksApp({ RenderView renderViewOverride }) : super(renderViewOverride: renderViewOverride);
 
+  NavigationState _navState = new NavigationState([
+    new Route(name: '/', builder: (navigator) => new StockHome(navigator)),
+    new Route(name: '/settings', builder: (navigator) => new StockSettings(navigator)),
+  ]);
+
+  void onBack() {
+    if (_navState.hasPrevious()) {
+      setState(() {
+        _navState.pop();
+      });
+      return;
+    }
+    print ("Should exit app here");
+    // TODO(jackson): Need a way to invoke default back behavior here
+  }
+
   Widget build() {
-    return new Navigator(
-      routes: [
-        new Route(
-          name: '/', 
-          builder: (navigator) => new StockHome(navigator)
-        ),
-        new Route(
-          name: '/settings', 
-          builder: (navigator) => new StockSettings(navigator)
-        ),
-      ]
-    );
+    return new Navigator(_navState);
   }
 }
 
 void main() {
   print("starting stocks app!");
   App app = new StocksApp();
   WidgetAppView.appView.onFrame = () {
     // uncomment this for debugging:
     // WidgetAppView.appView.debugDumpRenderTree();
   };
 }