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

pkg/BUILD.gn

 # 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.
 
 import("../utils/create_timestamp.gni")
 
 template("make_third_party_pkg_files_stamp") {
   assert(defined(invoker.id), "Must define the stamp file id")
   path = rebase_path("../third_party/pkg")
   if (defined(invoker.path)) {
     path = invoker.path
   }
   id = invoker.id
   create_timestamp_file(target_name) {
     if (defined(invoker.pattern)) {
       pattern = invoker.pattern
     }
     path = path
-    new_base = "//"
     output = "$target_gen_dir/third_party_pkg_files_$id.stamp"
   }
 }
 
 make_third_party_pkg_files_stamp("make_third_party_pkg_files_0_stamp") {
   path = rebase_path(".")
   id = "0"
 }
 
 make_third_party_pkg_files_stamp("make_third_party_pkg_files_1_stamp") {
@@ skipping 26 matching lines @@
 
   inputs = stamp0_outputs + stamp1_outputs + stamp2_outputs + stamp3_outputs
 
   outputs = [
     "$root_gen_dir/pkg_files.stamp",
   ]
 
   script = "../tools/create_timestamp_file.py"
   args = [ rebase_path("$root_gen_dir/pkg_files.stamp") ]
 }