Fix windows build.

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2011, 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 os
 import shutil
@@ skipping 175 matching lines @@
   process.wait()
   if process.returncode != 0:
     raise Error(args[0] + " failed")
 
 
 def GClientRunHooks():
   Execute(['gclient', 'runhooks'])
 
 
 def RunhooksIfNeeded(host_os, mode, arch, target_os):
+  if host_os != 'linux':
+    return
   build_root = utils.GetBuildRoot(host_os)
   build_cookie_path = os.path.join(build_root, 'lastHooksTargetOS.txt')
 
   old_target_os = None
   try:
     with open(build_cookie_path) as f:
       old_target_os = f.read(1024)
   except IOError as e:
     pass
   if target_os != old_target_os:
@@ skipping 108 matching lines @@
         process.wait()
         if process.returncode != 0:
           print "BUILD FAILED"
           return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())