title: How to find out what element is going to be extended, while registering my own in Polymer
body: <p>I recently asked <a href="https://stackoverflow.com/questions/24157576/how-to-extend-multiple-elements-with-polymer">How to extend multiple elements with Polymer</a> and it turned out, in fact, you can't really. The idea was to create a web component that can be applied to different elements to "decorate" them.</p>
  
  <p>Addy Osmani <a href="https://stackoverflow.com/a/24162853/1531806">answered</a> this question with a few approaches to handle that use case.</p>
  
  <p>One of them was:</p>
  
  <blockquote>
    <p>The alternative (if you strictly want to do this all in one custom
    element, which imo, makes this less clean) is to do something like
    checking against the type of element being extended, which could
    either be done in the manner you <a href="https://github.com/mvaldetaro/twbs-buttons" rel="nofollow noreferrer">linked to</a> or by checking as part of
    your element registration process.</p>
  </blockquote>
  
  <p>Despite from fact that this approach <strong>might</strong> be less clean, my question is:</p>
  
  <p><strong>How</strong> I can find out what element is going to be extended while I'm registering my own?</p>