ES6: Add missing Set.prototype.keys function

src/collection-iterator.js

 // Copyright 2014 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 $Set = global.Set;
@@ skipping 69 matching lines @@
 }
 
 SetUpSetIterator();
 
 
 function ExtendSetPrototype() {
   %CheckIsBootstrapping();
 
   InstallFunctions($Set.prototype, DONT_ENUM, $Array(
     'entries', SetEntries,
+    'keys', SetValues,
     'values', SetValues
   ));
 
   %AddProperty($Set.prototype, symbolIterator, SetValues, DONT_ENUM);
 }
 
 ExtendSetPrototype();
 
 
 
@@ skipping 84 matching lines @@
   InstallFunctions($Map.prototype, DONT_ENUM, $Array(
     'entries', MapEntries,
     'keys', MapKeys,
     'values', MapValues
   ));
 
   %AddProperty($Map.prototype, symbolIterator, MapEntries, DONT_ENUM);
 }
 
 ExtendMapPrototype();