[d8 Workers] Make Worker prototype read-only

test/mjsunit/regress/regress-crbug-518747.js

Index: test/mjsunit/regress/regress-crbug-518747.js
diff --git a/test/mjsunit/regress/regress-crbug-503698.js b/test/mjsunit/regress/regress-crbug-518747.js
similarity index 81%
copy from test/mjsunit/regress/regress-crbug-503698.js
copy to test/mjsunit/regress/regress-crbug-518747.js
index 415d1bc81be5ac46fb358ce8ec681b1e5c276e07..f1787c4c4b840d7a1c412c4dd64d15efd6620ee2 100644
--- a/test/mjsunit/regress/regress-crbug-503698.js
+++ b/test/mjsunit/regress/regress-crbug-518747.js
@@ -2,8 +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) {
+  Worker.prototype = 12;
   var __v_6 = new Worker('');
+  __v_6.postMessage([]);
 }