[es6] Fix length property of collection constructors

src/weak-collection.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 "use strict";
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 67 matching lines @@
   return %WeakCollectionDelete(this, key);
 }
 
 
 // -------------------------------------------------------------------
 
 function SetUpWeakMap() {
   %CheckIsBootstrapping();
 
   %SetCode($WeakMap, WeakMapConstructor);
+  %FunctionSetLength($WeakMap, 0);
   %FunctionSetPrototype($WeakMap, new $Object());
   %AddNamedProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);
   %AddNamedProperty(
       $WeakMap.prototype, symbolToStringTag, "WeakMap", DONT_ENUM | READ_ONLY);
 
   // Set up the non-enumerable functions on the WeakMap prototype object.
   InstallFunctions($WeakMap.prototype, DONT_ENUM, $Array(
     "get", WeakMapGet,
     "set", WeakMapSet,
     "has", WeakMapHas,
@@ skipping 57 matching lines @@
   return %WeakCollectionDelete(this, value);
 }
 
 
 // -------------------------------------------------------------------
 
 function SetUpWeakSet() {
   %CheckIsBootstrapping();
 
   %SetCode($WeakSet, WeakSetConstructor);
+  %FunctionSetLength($WeakSet, 0);
   %FunctionSetPrototype($WeakSet, new $Object());
   %AddNamedProperty($WeakSet.prototype, "constructor", $WeakSet, DONT_ENUM);
   %AddNamedProperty(
       $WeakSet.prototype, symbolToStringTag, "WeakSet", DONT_ENUM | READ_ONLY);
 
   // Set up the non-enumerable functions on the WeakSet prototype object.
   InstallFunctions($WeakSet.prototype, DONT_ENUM, $Array(
     "add", WeakSetAdd,
     "has", WeakSetHas,
     "delete", WeakSetDelete
   ));
 }
 
 SetUpWeakSet();