title: LitElement get updated attribute value when property set to new value
body: <p>I want attribute should update when property was set to new value.
  so what I did is to call the <code>element.requestUpdate()</code> so that I can get the updated attribute, but requestUpdate is async.
  Is there other way to get the updated attribute without calling requestUpdate? 
  Stackblitz URL 
  => <a href="https://stackblitz.com/edit/typescript-nojnqa" rel="nofollow noreferrer">https://stackblitz.com/edit/typescript-nojnqa</a></p>
  
  <p><a href="https://i.stack.imgur.com/t73KE.png" rel="nofollow noreferrer">LitElement get updated attribute</a></p>