Fix cluster-fuzz found regression in d8 Workers.

test/mjsunit/regress/regress-crbug-504729.js

Index: test/mjsunit/regress/regress-crbug-504729.js
diff --git a/test/mjsunit/regress/regress-crbug-503698.js b/test/mjsunit/regress/regress-crbug-504729.js
similarity index 60%
copy from test/mjsunit/regress/regress-crbug-503698.js
copy to test/mjsunit/regress/regress-crbug-504729.js
index 274eb5a99e9bc392107d933400f1ff2a3aab0e13..2a790542013bc9f7240982cb576b45e4d5a10afd 100644
--- a/test/mjsunit/regress/regress-crbug-503698.js
+++ b/test/mjsunit/regress/regress-crbug-504729.js
@@ -2,9 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --invoke-weak-callbacks
-
 if (this.Worker) {
-  function __f_1() {}
-  var __v_6 = new Worker(__f_1);
+  Function.prototype.toString = "foo";
+  function __f_7() {}
+  assertThrows(function() { var __v_5 = new Worker(__f_7); });
 }