Fix issue 1615 . Because frog doesn't really produce an artifact,

frog/scripts/list_frog_files.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.
 
 # traverses frog and lists all possible sources that should be marked as
 # dependencies for building frog/frogsh.
 
 import os
 import sys
 
 def main(argv):
-  for root, directories, files in os.walk(os.curdir):
+  if len(argv) == 1:
+    dir = os.curdir
+  else:
+    dir = argv[1]
+  for root, directories, files in os.walk(dir):
     if root == os.curdir:
       directories[0:] = ['leg', 'lib',
                          os.path.join('..', 'client', 'dom', 'frog'),
                          os.path.join('..', 'client', 'html', 'release')]
     for filename in files:
       if filename.endswith('.dart') or filename.endswith('.js'):
         print os.path.relpath(os.path.join(root, filename))
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))