[Fuchsia] Update prebuilt SDK sha only after a successful download

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.
 
@@ skipping 54 matching lines @@
     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)
+    with open(local_sha_path, 'w') as fp:
+      fp.write(remote_sha)
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))