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

tools/list_dart_files.py

 #!/usr/bin/env python
 # Copyright (c) 2016, 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 Dart source files.
 
+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_dart_files.py <directory> <root directory pattern>
+  python tools/list_dart_files.py {absolute, relative} <directory> <pattern>
 """
 
 import os
 import re
 import sys
 
 
 def main(argv):
-  directory = argv[1]
+  mode = argv[1]
+  if mode not in ['absolute', 'relative']:
+    raise Exception("First argument must be 'absolute' or 'relative'")
+  directory = argv[2]
+  if mode in 'absolute' and not os.path.isabs(directory):
+    directory = os.path.realpath(directory)
 
   pattern = None
-  if len(argv) > 2:
-    pattern = re.compile(argv[2])
+  if len(argv) > 3:
+    pattern = re.compile(argv[3])
 
   for root, directories, files in os.walk(directory):
     # We only care about actual source files, not generated code or tests.
     for skip_dir in ['.git', 'gen', 'test']:
       if skip_dir in directories:
         directories.remove(skip_dir)
 
     # If we are looking at the root directory, filter the immediate
     # subdirectories by the given pattern.
     if pattern and root == directory:
       directories[:] = filter(pattern.match, directories)
 
     for filename in files:
       if filename.endswith('.dart') and not filename.endswith('_test.dart'):
-        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, '/')
         print fullname
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))