Version 3.5.6.

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 15 matching lines @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // const $Object = global.Object;
 const $WeakMap = global.WeakMap;
 
 // -------------------------------------------------------------------
 
-// Set the WeakMap function and constructor.
-%SetCode($WeakMap, function(x) {
+function WeakMapConstructor() {
   if (%_IsConstructCall()) {
     %WeakMapInitialize(this);
   } else {
     return new $WeakMap();
   }
-});
+}
 
 
 function WeakMapGet(key) {
   if (!IS_SPEC_OBJECT(key)) {
     throw %MakeTypeError('invalid_weakmap_key', [this, key]);
   }
   return %WeakMapGet(this, key);
 }
 
 
@@ skipping 21 matching lines @@
     %WeakMapSet(this, key, void 0);
     return true;
   } else {
     return false;
   }
 }
 
 // -------------------------------------------------------------------
 
 function SetupWeakMap() {
+  // Setup the WeakMap constructor function.
+  %SetCode($WeakMap, WeakMapConstructor);
+
+  // Setup the WeakMap prototype object.
+  %FunctionSetPrototype($WeakMap, new $WeakMap());
+
   // Setup the non-enumerable functions on the WeakMap prototype object.
   InstallFunctionsOnHiddenPrototype($WeakMap.prototype, DONT_ENUM, $Array(
     "get", WeakMapGet,
     "set", WeakMapSet,
     "has", WeakMapHas,
     "delete", WeakMapDelete
   ));
 }
 
 
 SetupWeakMap();