Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/checkbox.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 '../layout.dart';
 import 'button_base.dart';
 import 'dart:sky' as sky;
 
 typedef void ValueChanged(value);
@@ skipping 67 matching lines @@
                 style: checked ? _checkedStyle : _uncheckedStyle
               )
             ]
           )
         ]
       ),
       onGestureTap: _handleClick
     );
   }
 }