Behaviors may not run `registered` if legacy element is subclassed

[dfreedm]

Root cause of https://github.com/PolymerElements/iron-a11y-keys-behavior/issues/93

In this scenario:

<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>

x-bar will have registered called on it, and x-foo will not. This means that fizz will only be set on x-bar, and not on x-foo's class, which is causing the error in iron-a11y-keys-behavior.

[dfreedm]

This is caused by moving behavior processing to be more like 1.x, which happened in 3.2.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!