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

utils/dartdoc/dartdoc.gyp

 # Copyright (c) 2014, 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.
 
 {
   'targets': [
     {
       'target_name': 'dartdoc',
       'type': 'none',
       'dependencies': [
         '../../runtime/dart-runtime.gyp:dart',
       ],
       'actions': [
         {
           'action_name': 'generate_dartdoc_snapshot',
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '../../sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart',
-            '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../third_party/pkg/dartdoc"])',
+            '<!@(["python", "../../tools/list_dart_files.py", "relative", '
+                '"../../third_party/pkg/dartdoc"])',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/dartdoc.dart.snapshot',
           ],
           'action': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '--packages=../../.packages',
             '--snapshot=<(SHARED_INTERMEDIATE_DIR)/dartdoc.dart.snapshot',
             '../../third_party/pkg/dartdoc/bin/dartdoc.dart',
           ],
         },
       ],
     },
   ],
-}
+}