Rationalize Dart mojo and sky package structure

sky/sdk/lib/download_material_design_icons

 #!/usr/bin/env python
 # Copyright (c) 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.
 
 import os
 import subprocess
 import urllib2
 
 sky_lib_dir = os.path.dirname(os.path.abspath(__file__))
 assets_dir = os.path.join(sky_lib_dir, 'assets')
 sha1_path = os.path.join(assets_dir, 'material-design-icons.sha1')
 
 with open(sha1_path, 'r') as f:
     sha1 = f.read()
 
 tgz_path = os.path.join(assets_dir, 'material-design-icons.tgz')
 url = 'https://storage.googleapis.com/mojo/material-design-icons/%s' % sha1
 response = urllib2.urlopen(url)
 
 with open(tgz_path, 'wb') as f:
     f.write(response.read())
 
 output_path = os.path.join(assets_dir, tgz_path)
 subprocess.call([
     'tar', '-xzf', output_path, '-C', assets_dir
 ])
 
 os.unlink(tgz_path)