Rationalize Dart mojo and sky package structure

mojo/public/dart/lib/src/event_stream.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 MojoEventStream extends Stream<List<int>> {
   // The underlying Mojo handle.
   MojoHandle _handle;
 
@@ skipping 229 matching lines @@
   void handleWrite() {}
 
   MojoMessagePipeEndpoint get endpoint => _endpoint;
   bool get isOpen => _isOpen;
   bool get isInHandler => _isInHandler;
   bool get isBound => _endpoint != null;
 
   String toString() => "MojoEventStreamListener("
       "isOpen: $isOpen, isBound: $isBound, endpoint: $_endpoint)";
 }