Rationalize Dart mojo and sky package structure

sky/sdk/bin/init.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.
 
 /*
 * This script should be invoked via 'pub run' after 'pub get':
 * $ pub run sky:init
 * NOTE: The 'dart' executable must be on your $PATH for this script to work.
 */
 
 import 'dart:io';
 
 main(List<String> arguments) {
   ProcessResult result = Process.runSync('dart', ['-p', 'packages', 'packages/mojom/generate.dart']);
   stdout.write(result.stdout);
   stderr.write(result.stderr);
 }