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

utils/dartanalyzer/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("//build/compiled_action.gni")
 import("../application_snapshot.gni")
 
 group("dartanalyzer") {
   deps = [
     ":generate_dartanalyzer_snapshot",
     ":generate_summary_spec",
     ":generate_summary_strong",
   ]
 }
 
 analyzer_files = exec_script("../../tools/list_dart_files.py",
-                             [ rebase_path("../../pkg/analyzer") ],
+                             [ "absolute", rebase_path("../../pkg/analyzer") ],
                              "list lines")
 
 application_snapshot("generate_dartanalyzer_snapshot") {
   main_dart = "../../pkg/analyzer_cli/bin/analyzer.dart"
   training_args = [
     "--dart-sdk=" + rebase_path("../../sdk"),
     rebase_path("../../tests/language/first_test.dart"),
   ]
   name = "dartanalyzer"
   cli_files = exec_script("../../tools/list_dart_files.py",
-                          [ rebase_path("../../pkg/analyzer_cli") ],
+                          [ "absolute", rebase_path("../../pkg/analyzer_cli") ],
                           "list lines")
   inputs = cli_files + analyzer_files
 }
 
 sdk_lib_files = exec_script("../../tools/list_dart_files.py",
-                            [ rebase_path("../../sdk/lib") ],
+                            [ "absolute", rebase_path("../../sdk/lib") ],
                             "list lines")
 
 template("generate_summary") {
   assert(defined(invoker.type), "Must specify the summary type")
   type = invoker.type
   assert(type == "spec" || type == "strong")
   compiled_action(target_name) {
     tool = "../../runtime/bin:dart"
     inputs = sdk_lib_files + analyzer_files
 
@@ skipping 17 matching lines @@
   }
 }
 
 generate_summary("generate_summary_spec") {
   type = "spec"
 }
 
 generate_summary("generate_summary_strong") {
   type = "strong"
 }