Fix back button behavior when returning from settings screen

sky/sdk/example/stocks/lib/stock_home.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/editing/input.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/drawer.dart';
 import 'package:sky/widgets/drawer_header.dart';
 import 'package:sky/widgets/floating_action_button.dart';
 import 'package:sky/widgets/icon.dart';
@@ skipping 131 matching lines @@
         new MenuItem(
           icon: 'action/thumb_down',
           onPressed: () => _handleStockModeChange(StockMode.pessimistic),
           children: [
             new Flexible(child: new Text('Pessimistic')),
             new Radio(value: StockMode.pessimistic, groupValue: stockMode, onChanged: _handleStockModeChange)
           ]),
         new MenuDivider(),
         new MenuItem(
           icon: 'action/settings',
-          onPressed: () => navigator.pushNamed('/settings'),
+          onPressed: _handleShowSettings,
           children: [new Text('Settings')]),
         new MenuItem(
           icon: 'action/help',
           children: [new Text('Help & Feedback')])
      ]
     );
   }
 
+  void _handleShowSettings() {
+    assert(navigator.currentRoute.name == '/drawer');
+    navigator.pop();
+    assert(navigator.currentRoute.name == '/');
+    navigator.pushNamed('/settings');
+  }
+
   void _handleOpenDrawer() {
     _drawerController.open();
     navigator.pushState("/drawer", (_) {
       _drawerController.close();
     });
   }
 
   Widget buildToolBar() {
     return new ToolBar(
         left: new IconButton(
@@ skipping 114 matching lines @@
         body: buildTabNavigator(),
         snackBar: buildSnackBar(),
         floatingActionButton: buildFloatingActionButton(),
         drawer: _drawerShowing ? buildDrawer() : null
       ),
     ];
     addMenuToOverlays(overlays);
     return new Stack(overlays);
   }
 }