---------------------------------------------------------------------------- Iterator related spec functions. ES6 7.4.1 GetIterator(obj, method)
function GetIterator(obj, method) {
  if (IS_UNDEFINED(method)) {
    method = obj[iteratorSymbol];
  }
  if (!IS_CALLABLE(method)) {
    throw MakeTypeError(kNotIterable, obj);
  }
  var iterator = %_Call(method, obj);
  if (!IS_RECEIVER(iterator)) {
    throw MakeTypeError(kNotAnIterator, iterator);
  }
  return iterator;
}