Rationalize Dart mojo and sky package structure

mojo/public/dart/lib/src/message.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 bindings;
 
 class MessageHeader {
   static const int kSimpleMessageSize = 16;
   static const int kSimpleMessageVersion = 0;
   static const int kMessageWithRequestIdSize = 24;
@@ skipping 107 matching lines @@
   Message get payload {
     if (_payload == null) {
       var truncatedBuffer = new ByteData.view(buffer.buffer, header.size);
       _payload = new Message(truncatedBuffer, handles);
     }
     return _payload;
   }
 
   String toString() => "ServiceMessage($header, $_payload)";
 }