Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/elements/sky-drawer.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="shadow.sky" as="shadow" />
 <import src="sky-element.sky" />
 <import src="sky-scrollable.sky" />
 
 <sky-element attributes="level:number">
@@ skipping 146 matching lines @@
       begin: currentPosition,
       end: targetPosition,
       duration: duration,
       curve: linear);
   }
 }
 
 _init(script) => register(script, SkyDrawer);
 </script>
 </sky-element>