Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/modal_overlay.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';
 
 class ModalOverlay extends Component {
   static final Style _style = new Style('''
     position: absolute;
     top: 0;
     left: 0;
     bottom: 0;
     right: 0;''');
 
   List<UINode> children;
   GestureEventListener onDismiss;
 
   ModalOverlay({ Object key, this.children, this.onDismiss }) : super(key: key);
 
   UINode build() {
     return new EventListenerNode(
       new Container(
         style: _style,
         children: children),
       onGestureTap: onDismiss);
   }
 }