Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/button_base.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';
 
 abstract class ButtonBase extends Component {
   bool highlight = false;
 
   ButtonBase({ Object key }) : super(key: key);
@@ skipping 18 matching lines @@
     setState(() {
       highlight = false;
     });
   }
   void _handlePointerCancel(_) {
     setState(() {
       highlight = false;
     });
   }
 }