Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/elements/material-element.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" />
 <import src="sky-ink-splash.sky" />
 <script>
 import "dart:sky";
 
@@ skipping 42 matching lines @@
       removeTransformHack();
   }
 
   void _handleScrollStart(GestureEvent event) {
     for (SkyInkSplash splash in _activeSplashes) {
       splash.cancel();
     }
   }
 }
 </script>