Implement the `RegExp.prototype.flags` getter

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index bf47a45c9d21583b4fadb11b59fdaa3b6e23995c..56d4ddb504d9aed1826df310e60e3c4f114cff77 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -52,6 +52,7 @@ var kMessages = {
   apply_wrong_args:              ["Function.prototype.apply: Arguments list has wrong type"],
   toMethod_non_function:         ["Function.prototype.toMethod was called on ", "%0", ", which is a ", "%1", " and not a function"],
   toMethod_non_object:           ["Function.prototype.toMethod: home object ", "%0", " is not an object"],
+  flags_getter_non_object:       ["RegExp.prototype.flags getter called on non-object ", "%0"],
   invalid_in_operator_use:       ["Cannot use 'in' operator to search for '", "%0", "' in ", "%1"],
   instanceof_function_expected:  ["Expecting a function in instanceof check, but got ", "%0"],
   instanceof_nonobject_proto:    ["Function has non-object prototype '", "%0", "' in instanceof check"],