Implement the `RegExp.prototype.flags` getter

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
 import("//build/module_args/v8.gni")
 
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
@@ skipping 225 matching lines @@
   sources = [
     "src/macros.py",
     "src/proxy.js",
     "src/generator.js",
     "src/harmony-string.js",
     "src/harmony-array.js",
     "src/harmony-array-includes.js",
     "src/harmony-typedarray.js",
     "src/harmony-classes.js",
     "src/harmony-tostring.js",
-    "src/harmony-templates.js"
+    "src/harmony-templates.js",
+    "src/harmony-regexp.js"
   ]
 
   outputs = [
     "$target_gen_dir/experimental-libraries.cc"
   ]
 
   args = [
     rebase_path("$target_gen_dir/experimental-libraries.cc", root_build_dir),
     "EXPERIMENTAL",
   ] + rebase_path(sources, root_build_dir)
@@ skipping 1195 matching lines @@
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
     ]
   }
 
   direct_dependent_configs = [ ":external_config" ]
 }
 
 }