private function instantiateDefaultRoles()
    {
        // this method can be removed in favor of `yii\rbac\BaseManager::getDefaultRoles()` in case
        // extension dependency on `yii2` is raised up to 2.0.12
        $result = [];
        foreach ($this->defaultRoles as $roleName) {
            $result[$roleName] = $this->createRole($roleName);
        }
        return $result;
    }