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

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index 095bc948a2f1a6fab5767e152313530bbb477af1..46ad21818e78f4940dd8594f34b91c54f1da6b87 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -1270,7 +1270,11 @@ RUNTIME_FUNCTION(Runtime_CreateIterResultObject) {
   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;
 }