Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/elements/sky-menu-divider.sky

 <!--
 // 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 src="sky-element.sky" />
 
 <sky-element>
 <template>
   <style>
     :host {
       margin: 8px 0;
       border-bottom: 1px solid rgba(0, 0, 0, 0.12);
     }
   </style>
 </template>
 <script>
 import "dart:sky";
 
 @Tagname('sky-menu-divider')
 class SkyMenuDivider extends SkyElement {
 }
 
 _init(script) => register(script, SkyMenuDivider);
 </script>
 </sky-element>