Fix only_in_release_mode.py's subcommand paths on Windows.

tools/only_in_release_mode.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2013, 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.
 
 """
 Wrapper around a build action that should only be executed in release mode.
 
 The mode is defined via an environment varible DART_BUILD_MODE.
@@ skipping 10 matching lines @@
 import os
 import subprocess
 import sys
 
 
 def Main():
   # Throws an error if '--' is not in the argument list.
   separator_index = sys.argv.index('--')
   outputs = sys.argv[1:separator_index]
   arguments = sys.argv[separator_index + 1:]
+  arguments[0] = os.path.normpath(arguments[0])
   mode = os.getenv('DART_BUILD_MODE', default='release')
   if mode != 'release':
     print >> sys.stderr, 'Not running %s in mode=%s' % (arguments, mode)
     for output in outputs:
       with open(output, 'w'):
         # Create an empty file to ensure that we don't rerun this
         # command unnecessarily.
         pass
     return 0
   else:
     try:
       subprocess.check_call(arguments)
     except subprocess.CalledProcessError as e:
       return e.returncode
     return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())