Rationalize Dart mojo and sky package structure

mojo/public/dart/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);
-  }
-}