[es6] Iterators and generators should "extend" %IteratorPrototype%

src/iterator-prototype.js

+// Copyright 2015 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.
+
+var $iteratorPrototype;
+
+(function(global, shared, exports) {
+  "use strict";
+  %CheckIsBootstrapping();
+
+  var GlobalObject = global.Object;
+
+  // 25.1.2.1 %IteratorPrototype% [ @@iterator ] ( )
+  function IteratorPrototypeIterator() {
+    return this;
+  }
+
+  $setFunctionName(IteratorPrototypeIterator, symbolIterator);
+  %AddNamedProperty($iteratorPrototype, symbolIterator,
+      IteratorPrototypeIterator, DONT_ENUM);
+})