Add ES2015 RegExp full subclassing semantics behind a flag

test/mjsunit/harmony/regexp-change-exec.js

Index: test/mjsunit/harmony/regexp-change-exec.js
diff --git a/test/message/regress/regress-4829-1.js b/test/mjsunit/harmony/regexp-change-exec.js
similarity index 50%
copy from test/message/regress/regress-4829-1.js
copy to test/mjsunit/harmony/regexp-change-exec.js
index 1ad5feda8cb96cd24e4c47f7407c9699d150c327..4c9757e3d50094d116123e8d14855775b497e679 100644
--- a/test/message/regress/regress-4829-1.js
+++ b/test/mjsunit/harmony/regexp-change-exec.js
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-function tag() {}
+// Flags: --harmony-regexp-exec
 
-tag(tag`\xyy`);
+class MyError extends Error { }
+RegExp.prototype.exec = () => { throw new MyError() };
+assertThrows(() => "foo".match(/bar/), MyError);