Lock the prototype of internal classes.

src/weakmap.js

Index: src/weakmap.js
diff --git a/src/weakmap.js b/src/weakmap.js
index 3d261e5afda2e18502f3af41661b96cdd977258b..6c15e30fb6f3b6dfb82d565e1293818f3221165a 100644
--- a/src/weakmap.js
+++ b/src/weakmap.js
@@ -80,7 +80,8 @@ function WeakMapDelete(key) {
 
 // -------------------------------------------------------------------
 
-function SetupWeakMap() {
+(function () {
+  %CheckIsBootstrapping();
   // Set up the WeakMap constructor function.
   %SetCode($WeakMap, WeakMapConstructor);
 
@@ -97,7 +98,4 @@ function SetupWeakMap() {
     "has", WeakMapHas,
     "delete", WeakMapDelete
   ));
-}
-
-
-SetupWeakMap();
+})();