title: change prototype of a custom element
body: <p>If I create a custom element as follows</p>
  
  <pre><code>document.register('bar-foo', { prototype: { .... } }); // or document.registerElement
  </code></pre>
  
  <p>Now, after this I would like to add something the the prototype of that element.</p>
  
  <p>For example, I can do this with the 'div' element:</p>
  
  <pre><code>HTMLDivElement.prototype.bar = function() {};
  document.createElement('div').bar();
  </code></pre>
  
  <p>How would I do that with my custom element 'bar-foo' ?</p>