Lock the prototype of internal classes.

src/weakmap.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 62 matching lines @@
   if (!IS_UNDEFINED(%WeakMapGet(this, key))) {
     %WeakMapSet(this, key, void 0);
     return true;
   } else {
     return false;
   }
 }
 
 // -------------------------------------------------------------------
 
-function SetupWeakMap() {
+(function () {
+  %CheckIsBootstrapping();
   // Set up the WeakMap constructor function.
   %SetCode($WeakMap, WeakMapConstructor);
 
   // Set up the WeakMap prototype object.
   %FunctionSetPrototype($WeakMap, new $WeakMap());
 
   // Set up the constructor property on the WeakMap prototype object.
   %SetProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);
 
   // Set up the non-enumerable functions on the WeakMap prototype object.
   InstallFunctionsOnHiddenPrototype($WeakMap.prototype, DONT_ENUM, $Array(
     "get", WeakMapGet,
     "set", WeakMapSet,
     "has", WeakMapHas,
     "delete", WeakMapDelete
   ));
-}
-
-
-SetupWeakMap();
+})();