[es6] Introduce a dedicated JSIteratorResult type.

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.
 
 var $mapEntries;
 var $mapIteratorNext;
 var $setIteratorNext;
 var $setValues;
 
 (function(global, utils) {
@@ skipping 14 matching lines @@
 }
 
 
 function SetIteratorNextJS() {
   if (!IS_SET_ITERATOR(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'Set Iterator.prototype.next', this);
   }
 
   var value_array = [UNDEFINED, UNDEFINED];
-  var entry = {value: value_array, done: false};
+  var result = %_CreateIterResultObject(value_array, false);
   switch (%SetIteratorNext(this, value_array)) {
     case 0:
-      entry.value = UNDEFINED;
-      entry.done = true;
+      result.value = UNDEFINED;
+      result.done = true;
       break;
     case ITERATOR_KIND_VALUES:
-      entry.value = value_array[0];
+      result.value = value_array[0];
       break;
     case ITERATOR_KIND_ENTRIES:
       value_array[1] = value_array[0];
       break;
   }
 
-  return entry;
+  return result;
 }
 
 
 function SetEntries() {
   if (!IS_SET(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'Set.prototype.entries', this);
   }
   return new SetIterator(this, ITERATOR_KIND_ENTRIES);
 }
@@ skipping 37 matching lines @@
 }
 
 
 function MapIteratorNextJS() {
   if (!IS_MAP_ITERATOR(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'Map Iterator.prototype.next', this);
   }
 
   var value_array = [UNDEFINED, UNDEFINED];
-  var entry = {value: value_array, done: false};
+  var result = %_CreateIterResultObject(value_array, false);
   switch (%MapIteratorNext(this, value_array)) {
     case 0:
-      entry.value = UNDEFINED;
-      entry.done = true;
+      result.value = UNDEFINED;
+      result.done = true;
       break;
     case ITERATOR_KIND_KEYS:
-      entry.value = value_array[0];
+      result.value = value_array[0];
       break;
     case ITERATOR_KIND_VALUES:
-      entry.value = value_array[1];
+      result.value = value_array[1];
       break;
     // ITERATOR_KIND_ENTRIES does not need any processing.
   }
 
-  return entry;
+  return result;
 }
 
 
 function MapEntries() {
   if (!IS_MAP(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'Map.prototype.entries', this);
   }
   return new MapIterator(this, ITERATOR_KIND_ENTRIES);
 }
@@ skipping 34 matching lines @@
   'keys', MapKeys,
   'values', MapValues
 ]);
 
 %AddNamedProperty(GlobalMap.prototype, iteratorSymbol, MapEntries, DONT_ENUM);
 
 $mapEntries = MapEntries;
 $mapIteratorNext = MapIteratorNextJS;
 
 })