[Effen] fix some dart warnings (mostly unused imports)

sky/framework/components/scaffold.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 '../fn.dart';
-import 'action_bar.dart';
 import 'drawer.dart';
 import 'floating_action_button.dart';
 import 'package:sky/framework/theme/typography.dart' as typography;
 
 class Scaffold extends Component {
   static final Style _style = new Style('''
     ${typography.typeface};
     ${typography.black.body1};''');
 
   static final Style _mainStyle = new Style('''
@@ skipping 44 matching lines @@
 
     if (drawer != null)
       children.add(new StyleNode(drawer, _drawerStyle));
 
     if (overlays != null)
       children.addAll(overlays);
 
     return new Container(style: _style, children: children);
   }
 }