Refactor stateful parts of Component into StatefulComponent

sky/sdk/lib/widgets/navigator.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 'basic.dart';
 
 typedef Widget Builder(Navigator navigator, RouteBase route);
 
 abstract class RouteBase {
   RouteBase({ this.name });
@@ skipping 56 matching lines @@
 
   void pop() {
     if (historyIndex > 0) {
       history[historyIndex].popState();
       history.removeLast();
       historyIndex--;
     }
   }
 }
 
-class Navigator extends Component {
+class Navigator extends StatefulComponent {
 
-  Navigator(this.state, { String key }) : super(key: key, stateful: true);
+  Navigator(this.state, { String key }) : super(key: key);
 
   NavigationState state;
 
   void syncFields(Navigator source) {
     state = source.state;
   }
 
   RouteBase get currentRoute => state.currentRoute;
 
   void pushState(String name, Function callback) {
@@ skipping 20 matching lines @@
   void pop() {
     setState(() {
       state.pop();
     });
   }
 
   Widget build() {
     return state.currentRoute.build(this, state.currentRoute);
   }
 }