public function offsetGet($key) {

    $resolved_key = $this->arguments_spec->normalizeOptionName($key);
    if ($resolved_key === null) {
      if (isset($this->merged_arg_values[$key])) {
        return parent::offsetGet($key);
      }
    }

    if (isset($this->merged_arg_values[$resolved_key])) {
      return parent::offsetGet($resolved_key);
    }

    return null;
  }