title: Custom Elements Illegal Constructor in FireFox
body: <p>I have a CustomElement with the following constructor:</p>
  
  <pre><code>export default class SomeCustomElement extends HTMLElement {
      constructor(templateId) {
          super();
          this.insertTemplateInstance(templateId);
      }
      ...
  }
  </code></pre>
  
  <p>I can register that Element in Chrome without any Problems. </p>
  
  <p>But using Firefox with the polyfill loaded by <code>webcomponents-loader.js</code> from <a href="https://github.com/webcomponents/webcomponentsjs" rel="nofollow noreferrer">https://github.com/webcomponents/webcomponentsjs</a> I get the ErrorMessage <code>TypeError: Illegal constructor</code> when calling <code>super()</code>.</p>
  
  <p>Does anybody know what is causing this? </p>
  
  <p>Some more Background:</p>
  
  <p>Registering of the custom Elements happens like this:</p>
  
  <pre><code>window.addEventListener("WebComponentsReady", function () {
      customElements.define(elementName, SomeCustomElement);
  });
  </code></pre>