Make list_files.py and list_dart_files.py return absolute paths for GN

tools/list_files.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 files whose name match a pattern.
 
+If the first argument is 'relative', the script produces paths relative to the
+current working directory. If the first argument is 'absolute', the script
+produces absolute paths.
+
 Usage:
-  python tools/list_files.py PATTERN DIRECTORY...
+  python tools/list_files.py {absolute, relative} PATTERN DIRECTORY...
 """
 
 import os
 import re
 import sys
 
 
 def main(argv):
-  pattern = re.compile(argv[1])
-  for directory in argv[2:]:
+  mode = argv[1]
+  if mode not in ['absolute', 'relative']:
+    raise Exception("First argument must be 'absolute' or 'relative'")
+  pattern = re.compile(argv[2])
+  for directory in argv[3:]:
+    if mode in 'absolute' and not os.path.isabs(directory):
+      directory = os.path.realpath(directory)
     for root, directories, files in os.walk(directory):
       if '.git' in directories:
         directories.remove('.git')
       for filename in files:
-        fullname = os.path.relpath(os.path.join(root, filename))
+        if mode in 'absolute':
+          fullname = os.path.join(directory, root, filename)
+        else:
+          fullname = os.path.relpath(os.path.join(root, filename))
         fullname = fullname.replace(os.sep, '/')
         if re.search(pattern, fullname):
           print fullname
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))