title: Constructor Optional Params
body: <p>Is there a way to set a constructor optional param?
  I mean something like:</p>
  
  <pre><code>User.fromData(this._name, 
    this._email, 
    this._token, 
    this._refreshToken,  
    this._createdAt,  
    this._expiresAt,  
    this._isValid,  
    {this.id});
  </code></pre>
  
  <p>It indicates that </p>
  
  <blockquote>
    <p>Named option parameters can't start with an underscore.</p>
  </blockquote>
  
  <p>But I need this field as private, so, I'm lost now.</p>