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

utils/dartdoc/dartdoc.gyp

Index: utils/dartdoc/dartdoc.gyp
diff --git a/utils/dartdoc/dartdoc.gyp b/utils/dartdoc/dartdoc.gyp
index dc5e414834cea0480b08013de2354409610cb0ad..e5327c0edc0adfa7206d7122e6dbcc41186d5a40 100644
--- a/utils/dartdoc/dartdoc.gyp
+++ b/utils/dartdoc/dartdoc.gyp
@@ -16,7 +16,8 @@
           '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',
@@ -31,4 +32,4 @@
       ],
     },
   ],
-}
+}