Back button should be able to close apps

sky/sdk/example/stocks/lib/main.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/theme/colors.dart' as colors;
 import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/default_text_style.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/theme.dart';
@@ skipping 14 matching lines @@
         builder: (navigator, route) => new StockHome(navigator, _stocks, optimismSetting, modeUpdater)
       ),
       new Route(
         name: '/settings',
         builder: (navigator, route) => new StockSettings(navigator, optimismSetting, backupSetting, settingsUpdater)
       ),
     ]);
   }
 
   void onBack() {
-    setState(() {
-      _navigationState.pop();
-    });
-    // TODO(jackson): Need a way to invoke default back behavior here
+    if (_navigationState.hasPrevious()) {
+      setState(() {
+        _navigationState.pop();
+      });
+    } else {
+      super.onBack();
+    }
   }
 
   StockMode optimismSetting = StockMode.optimistic;
   BackupMode backupSetting = BackupMode.disabled;
   void modeUpdater(StockMode optimism) {
     setState(() {
       optimismSetting = optimism;
     });
   }
   void settingsUpdater({ StockMode optimism, BackupMode backup }) {
@@ skipping 37 matching lines @@
           style: typography.error, // if you see this, you've forgotten to correctly configure the text style!
           child: new Navigator(_navigationState)
         )
      );
    }
 }
 
 void main() {
   runApp(new StocksApp());
 }