Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/popup_menu_item.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 'ink_well.dart';
 
 class PopupMenuItem extends Component {
   static final Style _style = new Style('''
     min-width: 112px;
     padding: 16px;''');
 
   List<UINode> children;
   double opacity;
 
   PopupMenuItem({ Object key, this.children, this.opacity}) : super(key: key);
 
   UINode build() {
     return new StyleNode(
       new InkWell(
         inlineStyle: opacity == null ? null : 'opacity: ${opacity}',
         children: children
       ),
       _style);
   }
 }