Use list_dart_files.py instead of list_files.py in GN build.

utils/create_timestamp.gni

Index: utils/create_timestamp.gni
diff --git a/utils/create_timestamp.gni b/utils/create_timestamp.gni
index eb711aebe904302a9dfc519803dc5b54b5d9d035..fce5c30e6a95d0aa1a7d902e9daf7ebaa997881c 100644
--- a/utils/create_timestamp.gni
+++ b/utils/create_timestamp.gni
@@ -5,7 +5,6 @@
 _dart_root = rebase_path("..")
 
 template("create_timestamp_file") {
-  assert(defined(invoker.pattern), "Must specify 'pattern'")
   assert(defined(invoker.path), "Must specify 'path'")
   assert(defined(invoker.output), "Must specify 'output'")
   new_base = "."
@@ -16,13 +15,16 @@ template("create_timestamp_file") {
   if (defined(invoker.current_base)) {
     current_base = invoker.current_base
   }
-  pattern = invoker.pattern
   path = invoker.path
   output = invoker.output
   action(target_name) {
+    list_args = [path]
+    if (defined(invoker.pattern)) {
+      list_args += [invoker.pattern]
+    }
     files = exec_script(
-        "$_dart_root/tools/list_files.py", [pattern, path], "list lines")
-    inputs = ["$_dart_root/tools/list_files.py"] +
+        "$_dart_root/tools/list_dart_files.py", list_args, "list lines")
+    inputs = ["$_dart_root/tools/list_dart_files.py"] +
              rebase_path(files, new_base, current_base)
     outputs = [ output ]
     script = "$_dart_root/tools/create_timestamp_file.py"