Rationalize Dart mojo and sky package structure

mojo/public/dart/lib/src/drain_data.dart

 // Copyright 2014 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.
 
 part of core;
 
 class DataPipeDrainer {
   MojoDataPipeConsumer _consumer;
   MojoEventStream _eventStream;
   List<ByteData> _dataList;
@@ skipping 52 matching lines @@
       }
     });
     return completer.future;
   }
 
   static Future<ByteData> drainHandle(MojoDataPipeConsumer consumer) {
     var drainer = new DataPipeDrainer(consumer);
     return drainer.drain();
   }
 }