Update clobber functionality to clobber runtime/{Debug,Release} on windows

tools/clean_output_directory.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, 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 optparse
 import shutil
 import sys
@@ skipping 36 matching lines @@
   if not ProcessOptions(options):
     parser.print_help()
     return 1
 
   # Delete the output for the targets for each requested configuration.
   for mode in options.mode:
     for arch in options.arch:
       build_root = utils.GetBuildRoot(HOST_OS, mode=mode, arch=arch)
       print "Deleting %s" % (build_root)
       shutil.rmtree(build_root, ignore_errors=True)
+      # On windows we have additional object files within the runtime library.
+      if HOST_OS == 'win32':
+        runtime_root = 'runtime/' + build_root
+        print "Deleting %s" % (runtime_root)
+        shutil.rmtree(runtime_root, ignore_errors=True)
   return 0
 
 if __name__ == '__main__':
   sys.exit(Main())