ES6: Add missing Set.prototype.keys function

test/mjsunit/harmony/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.
 
 // Flags: --harmony-collections --allow-natives-syntax
 
 
 (function TestSetIterator() {
   var s = new Set;
   var iter = s.values();
@@ skipping 19 matching lines @@
   var iter = s.values();
 
   assertEquals({value: 1, done: false}, iter.next());
   assertEquals({value: 2, done: false}, iter.next());
   assertEquals({value: 3, done: false}, iter.next());
   assertEquals({value: undefined, done: true}, iter.next());
   assertEquals({value: undefined, done: true}, iter.next());
 })();
 
 
+(function TestSetIteratorKeys() {
+  assertEquals(Set.prototype.keys, Set.prototype.values);
+})();
+
+
 (function TestSetIteratorEntries() {
   var s = new Set;
   s.add(1);
   s.add(2);
   s.add(3);
   var iter = s.entries();
 
   assertEquals({value: [1, 1], done: false}, iter.next());
   assertEquals({value: [2, 2], done: false}, iter.next());
   assertEquals({value: [3, 3], done: false}, iter.next());
@@ skipping 136 matching lines @@
 
 (function TestMapIteratorSymbol() {
   assertEquals(Map.prototype[Symbol.iterator], Map.prototype.entries);
   assertTrue(Map.prototype.hasOwnProperty(Symbol.iterator));
   assertFalse(Map.prototype.propertyIsEnumerable(Symbol.iterator));
 
   var iter = new Map().values();
   assertEquals(iter, iter[Symbol.iterator]());
   assertEquals(iter[Symbol.iterator].name, '[Symbol.iterator]');
 })();