dummy-element.html

<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="twitter-widgets.html">

<dom-module id="dummy-element">

  <template>

    <style>
      :host {
        display: block;
      }
    </style>

    <twitter-widgets>
      <a class="twitter-timeline" height="500" href="https://twitter.com/joaomgvieira">Tweets by joaomgvieira</a>
    </twitter-widgets>

  </template>

  <script>
    class DummyElement extends Polymer.Element {
      static get is() { return 'dummy-element'; }
    }
 
    window.customElements.define(DummyElement.is, DummyElement);
  </script>

</dom-module>