Rationalize Dart mojo and sky package structure

mojo/public/dart/lib/src/struct.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;
 
 abstract class Struct {
   final int encodedSize;
 
   Struct(this.encodedSize);
 
   void encode(Encoder encoder);
 
   Message serialize() {
     var encoder = new Encoder(encodedSize);
     encode(encoder);
     return encoder.message;
   }
 
   ServiceMessage serializeWithHeader(MessageHeader header) {
     var encoder = new Encoder(encodedSize + header.size);
     header.encode(encoder);
     encode(encoder);
     return new ServiceMessage(encoder.message, header);
   }
 }