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

tools/download_latest_dev_sdk.py

Index: tools/download_latest_dev_sdk.py
diff --git a/tools/download_latest_dev_sdk.py b/tools/download_latest_dev_sdk.py
index d39cae4ee5353109b25ad3d6cf5120aebe94462f..4865e103c0b9ef8eb32c55e56b78b6c33ee5fefd 100755
--- a/tools/download_latest_dev_sdk.py
+++ b/tools/download_latest_dev_sdk.py
@@ -72,13 +72,13 @@ def main(argv):
   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))