<script type="module">
  import {Polymer} from '@polymer/polymer/polymer-legacy.js';
  const behavior = {
    registered() {
      this.fizz = {};
    }
  };
  Polymer({
    is: 'x-foo',
    behaviors: [behavior]
  });

  class XBar extends customElements.get('x-foo') {}
  customElements.define(XBar.is, XBar);
</script>

<x-bar></x-bar>

<script>
  addEventListener('load', () => {
    const el = document.createElement('x-foo');
    document.body.appendChild(el);
  })
</script>