Implement the `RegExp.prototype.flags` getter

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index f13a3dab4e61965be20639c61cc82dc264dbb00c..c59f9964dff78b6c8662859da8c7058ba51cf456 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -2166,7 +2166,8 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_modules_natives[] = {NULL};
   static const char* harmony_scoping_natives[] = {NULL};
   static const char* harmony_object_literals_natives[] = {NULL};
-  static const char* harmony_regexps_natives[] = {NULL};
+  static const char* harmony_regexps_natives[] = {
+      "native harmony-regexp.js", NULL};
   static const char* harmony_arrow_functions_natives[] = {NULL};
   static const char* harmony_numeric_literals_natives[] = {NULL};
   static const char* harmony_tostring_natives[] = {"native harmony-tostring.js",