Refactor stateful parts of Component into StatefulComponent

sky/sdk/example/stocks/lib/stock_settings.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/checkbox.dart';
 import 'package:sky/widgets/switch.dart';
 import 'package:sky/widgets/flat_button.dart';
 import 'package:sky/widgets/dialog.dart';
 import 'package:sky/widgets/icon_button.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/menu_item.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/tool_bar.dart';
 
 import 'stock_types.dart';
 
 typedef void SettingsUpdater({
   StockMode optimism,
   BackupMode backup
 });
 
-class StockSettings extends Component {
+class StockSettings extends StatefulComponent {
 
-  StockSettings(this.navigator, this.optimism, this.backup, this.updater) : super(stateful: true);
+  StockSettings(this.navigator, this.optimism, this.backup, this.updater);
 
   Navigator navigator;
   StockMode optimism;
   BackupMode backup;
   SettingsUpdater updater;
 
   bool showModeDialog = false;
 
   void syncFields(StockSettings source) {
     navigator = source.navigator;
@@ skipping 99 matching lines @@
               _handleOptimismChanged(true);
               navigator.pop();
             }
           ),
         ]
       ));
     }
     return new Stack(layers);
   }
 }