Fixes dev SDK download script to sniff for the version Flutter wants

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, '..'))
+FUCHSIA_ROOT = os.path.realpath(os.path.join(DART_ROOT, '..'))
+FLUTTER_ROOT = os.path.join(FUCHSIA_ROOT, 'lib', 'flutter')
 
-DART_VERSION = '1.25.0-dev.7.0'
-BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/' + DART_VERSION +'/sdk'
+DEFAULT_DART_VERSION = 'latest'
+BASE_URL = 'http://gsdview.appspot.com/dart-archive/channels/dev/raw/%s/sdk/%s'
 
 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.
 def extract_file(zf, info, extract_dir):
   zf.extract( info.filename, path=extract_dir )
   out_path = os.path.join(extract_dir, info.filename)
   perm = info.external_attr >> 16L
   os.chmod(out_path, perm)
 
 def main(argv):
   host_os = host_os_for_sdk(HOST_OS)
   zip_file = ('dartsdk-%s-x64-release.zip' % HOST_OS)
   sha_file = zip_file + '.sha256sum'
   sdk_path = os.path.join(DART_ROOT, 'tools', 'sdks', host_os)
   local_sha_path = os.path.join(sdk_path, sha_file)
   remote_sha_path = os.path.join(sdk_path, sha_file + '.remote')
   zip_path = os.path.join(sdk_path, zip_file)
-  sha_url = BASE_URL + '/' + sha_file
-  zip_url = BASE_URL + '/' + zip_file
+
+  # If we're in a Fuchsia checkout with Flutter nearby, pull the same dev SDK
+  # version that Flutter says it wants. Otherwise, pull the latest dev SDK.
+  sdk_version_path = os.path.join(
+      FLUTTER_ROOT, 'bin', 'internal', 'dart-sdk.version')
+  sdk_version = DEFAULT_DART_VERSION
+  if os.path.isfile(sdk_version_path):
+    with open(sdk_version_path, 'r') as fp:
+      sdk_version = fp.read().strip()
+
+  sha_url = (BASE_URL % (sdk_version, sha_file))
+  zip_url = (BASE_URL % (sdk_version, zip_file))
 
   local_sha = ''
   if os.path.isfile(local_sha_path):
     with open(local_sha_path, 'r') as fp:
       local_sha = fp.read()
 
   remote_sha = ''
   urllib.urlretrieve(sha_url, remote_sha_path)
   with open(remote_sha_path, 'r') as fp:
     remote_sha = fp.read()
   os.remove(remote_sha_path)
 
   if local_sha == '' or local_sha != remote_sha:
     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))