Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/material.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 '../theme/shadows.dart';
 
 class Material extends Component {
   static final List<Style> _shadowStyle = [
     null,
     new Style('box-shadow: ${Shadow[1]}'),
     new Style('box-shadow: ${Shadow[2]}'),
     new Style('box-shadow: ${Shadow[3]}'),
     new Style('box-shadow: ${Shadow[4]}'),
     new Style('box-shadow: ${Shadow[5]}'),
   ];
 
   UINode content;
   int level;
 
   Material({ Object key, this.content, this.level: 0 }) : super(key: key);
 
   UINode build() {
     return new StyleNode(content, _shadowStyle[level]);
   }
 }