[runtime] We don't need an actual instance type for JSIteratorResult.

src/runtime/runtime-object.cc

 // 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.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/debug/debug.h"
 #include "src/isolate-inl.h"
@@ skipping 1252 matching lines @@
   return isolate->heap()->ToBoolean(result.FromJust());
 }
 
 
 // ES6 section 7.4.7 CreateIterResultObject ( value, done )
 RUNTIME_FUNCTION(Runtime_CreateIterResultObject) {
   HandleScope scope(isolate);
   DCHECK_EQ(2, args.length());
   CONVERT_ARG_HANDLE_CHECKED(Object, value, 0);
   CONVERT_ARG_HANDLE_CHECKED(Object, done, 1);
-  return *isolate->factory()->NewJSIteratorResult(value, done);
+  Handle<JSObject> result =
+      isolate->factory()->NewJSObjectFromMap(isolate->iterator_result_map());
+  result->InObjectPropertyAtPut(JSIteratorResult::kValueIndex, *value);
+  result->InObjectPropertyAtPut(JSIteratorResult::kDoneIndex, *done);
+  return *result;
 }
 
 
 RUNTIME_FUNCTION(Runtime_IsAccessCheckNeeded) {
   SealHandleScope shs(isolate);
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_CHECKED(Object, object, 0);
   return isolate->heap()->ToBoolean(object->IsAccessCheckNeeded());
 }
 
@@ skipping 13 matching lines @@
   DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(Object, o, 0);
   CONVERT_ARG_HANDLE_CHECKED(Object, properties, 1);
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
       isolate, o, JSReceiver::DefineProperties(isolate, o, properties));
   return *o;
 }
 
 }  // namespace internal
 }  // namespace v8