Pin the dev SDK version pulled by Fuchsia

tools/download_latest_dev_sdk.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project 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 downloads the latest dev SDK from
 # http://gsdview.appspot.com/dart-archive/channels/dev/raw/latest/sdk/
 # into tools/sdks/$HOST_OS/. It is intended to be invoked from Jiri hooks in
 # a Fuchsia checkout.
 
 import os
 import sys
 import zipfile
 import urllib
 import utils
 
 HOST_OS = utils.GuessOS()
 HOST_ARCH = utils.GuessArchitecture()
 SCRIPT_DIR = os.path.dirname(sys.argv[0])
 DART_ROOT = os.path.realpath(os.path.join(SCRIPT_DIR, '..'))
 
-BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/latest/sdk'
+DART_VERSION = '1.25.0-dev.7.0'
+BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/' + DART_VERSION +'/sdk'
 
 def host_os_for_sdk(host_os):
   if host_os.startswith('macos'):
     return 'mac'
   if host_os.startswith('win'):
     return 'windows'
   return host_os
 
 # Python's zipfile doesn't preserve file permissions during extraction, so we
 # have to do it manually.
@@ skipping 29 matching lines @@
     with open(local_sha_path, 'w') as fp:
       fp.write(remote_sha)
     print 'Downloading prebuilt Dart SDK from: ' + zip_url
     urllib.urlretrieve(zip_url, zip_path)
     with zipfile.ZipFile(zip_path, 'r') as zf:
       for info in zf.infolist():
         extract_file(zf, info, sdk_path)
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))