fix back button in stocks app by calling super.didMount()

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/sky_binding.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/widget.dart';
 
 import 'stock_data.dart';
 import 'stock_home.dart';
 import 'stock_settings.dart';
 
 class StocksApp extends App {
 
   StocksApp() {
     _navigationState = new NavigationState([
       new Route(
         name: '/', 
         builder: (navigator, route) => new StockHome(navigator, route, _stocks)
       ),
       new Route(
         name: '/settings',
         builder: (navigator, route) => new StockSettings(navigator)
       ),
     ]);
   }
 
   void didMount() {
+    super.didMount();
     new StockDataFetcher((StockData data) {
       setState(() {
         data.appendTo(_stocks);
       });
     });
   }
 
   final List<Stock> _stocks = [];
   NavigationState _navigationState;
 
@@ skipping 10 matching lines @@
 }
 
 void main() {
   print("starting stocks app!");
   runApp(new StocksApp());
   SkyBinding.instance.onFrame = () {
     // uncomment this for debugging:
     // SkyBinding.instance.debugDumpRenderTree();
   };
 }