Have list_pkg_directories.py print something.

dart/tools/list_pkg_directories.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.
 
-
 '''Tool for listing the directories under pkg, with their lib directories.
 Used in pkg.gyp. Lists all of the directories in the current directory
 which have a lib subdirectory.
 
 Usage:
   python tools/list_pkg_directories.py
 '''
+
 import os
+import sys
 
-paths = map(lambda x: x + '/lib', filter(os.path.isdir, os.listdir('.')))
-filter(lambda x: os.path.exists(x), paths)
+def main(argv):
+  paths = map(lambda x: x + '/lib', filter(os.path.isdir, os.listdir(argv[1])))
+  for lib in filter(lambda x: os.path.exists(x), paths):
+    print lib
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))