Version: 53.0.2775.0 (64-bit)

`this.attachShadow` is undefined in custom element constructor. I believe this code should work:

  customElements.define('x-foo', class extends HTMLElement {
    constructor() {
      super();
      let shadowRoot = this.attachShadow({mode: 'open'});
      shadowRoot.innerHTML = `
        <b>I'm in shadow dom!</b>
        <slot></slot>
      `;
    }
  });

Chromium is complaining it's undefined.