Always clobber sdk builds on dev/stable.

tools/bots/dart_sdk.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 # for details. 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.path
 import shutil
 import sys
 
@@ skipping 169 matching lines @@
     gsutil.upload(local_md5sum, remote_path + '.md5sum', public=True)
     local_sha256 = bot_utils.CreateSha256ChecksumFile(local_path,
                                                       mangled_filename)
     gsutil.upload(local_sha256, remote_path + '.sha256sum', public=True)
 
 def Run(command):
   print "Running %s" % ' '.join(command)
   return bot.RunProcess(command)
 
 if __name__ == '__main__':
+  if CHANNEL != bot_utils.Channel.BLEEDING_EDGE:
+    # We always clobber the bot, to make sure releases are build from scratch
+    bot.Clobber(force=True)
+
   CreateUploadSDK()
   if BUILD_OS == 'linux':
     CreateUploadVersionFile()
     CreateUploadAPIDocs()