Fix back button behavior when returning from settings screen

R=abarth@chromium.org, hixie

Review URL: https://codereview.chromium.org/1209423006.

sdk/example/stocks/lib/stock_home.dart

@@ -149,7 +149,7 @@ class StockHome extends StatefulComponent {
         new MenuDivider(),
         new MenuItem(
           icon: 'action/settings',
-          onPressed: () => navigator.pushNamed('/settings'),
+          onPressed: _handleShowSettings,
           children: [new Text('Settings')]),
         new MenuItem(
           icon: 'action/help',
@@ -158,6 +158,13 @@ class StockHome extends StatefulComponent {
     );
   }
 
+  void _handleShowSettings() {
+    assert(navigator.currentRoute.name == '/drawer');
+    navigator.pop();
+    assert(navigator.currentRoute.name == '/');
+    navigator.pushNamed('/settings');
+  }
+
   void _handleOpenDrawer() {
     _drawerController.open();
     navigator.pushState("/drawer", (_) {