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

sky/examples/widgets/navigation.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/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/transition.dart';
 import 'package:sky/widgets/raised_button.dart';
 
 List<Route> routes = [
@@ skipping 47 matching lines @@
         new RaisedButton(
           child: new Text('NO WAIT! GO BACK!'),
           onPressed: () => navigator.pop()
         )
       ])
     )
   )
 ];
 
 class NavigationExampleApp extends App {
+  NavigationState _navState = new NavigationState(routes);
+
   Widget build() {
-    return new Flex([new Navigator(routes: routes)]);
+    return new Flex([new Navigator(_navState)]);
   }
 }
 
 void main() {
   App app = new NavigationExampleApp();
 }