Rationalize Dart mojo and sky package structure

mojo/public/dart/lib/src/natives.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.
 
 part of internal;
 
 class MojoHandleNatives {
   static int register(
       Object eventStream, int handle) native "MojoHandle_Register";
   static int close(int handle) native "MojoHandle_Close";
@@ skipping 50 matching lines @@
   static List Create(int numBytes, int flags) native "MojoSharedBuffer_Create";
 
   static List Duplicate(
       int bufferHandle, int flags) native "MojoSharedBuffer_Duplicate";
 
   static List Map(Object buffer, int bufferHandle, int offset, int numBytes,
       int flags) native "MojoSharedBuffer_Map";
 
   static int Unmap(ByteData buffer) native "MojoSharedBuffer_Unmap";
 }