Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/components/animated_component.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 '../animation/animated_value.dart';
 import '../fn.dart';
 import 'dart:mirrors';
 
 abstract class AnimatedComponent extends Component {
   AnimatedComponent({ Object key }) : super(key: key, stateful: true);
@@ skipping 20 matching lines @@
     });
 
     onDidUnmount(() {
       if (subscription != null) {
         subscription.cancel();
         subscription = null;
       }
     });
   }
 }