Make list_files.py and list_dart_files.py return absolute paths

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.
 
 Usage:
   python tools/list_files.py PATTERN DIRECTORY...
 """
 
 import os
 import re
 import sys
 
 
 def main(argv):
   pattern = re.compile(argv[1])
   for directory in argv[2:]:
+    if 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))
+        fullname = os.path.join(directory, root, filename)
         fullname = fullname.replace(os.sep, '/')
         if re.search(pattern, fullname):
           print fullname
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))