message: Convert TabWidget to the Custom Elements v1 API
diff: @@ -60,7 +60,7 @@ export class TabWidget extends ThemeableElementBase {
  */
  static init(): void {
  if (registered === false) {
  - window.document.registerElement(TabWidget.TAG_NAME, {prototype: TabWidget.prototype});
  + window.customElements.define(TabWidget.TAG_NAME.toLowerCase(), TabWidget);
  registered = true;
  }
  }
  @@ -96,7 +96,9 @@ export class TabWidget extends ThemeableElementBase {
  /**
  * Custom Element 'created' life cycle hook.
  */
  - createdCallback() {
  + constructor() {
  + super();
  +
  this._initProperties();
  const shadow = this.attachShadow({ mode: 'open', delegatesFocus: false });
  const clone = this.createClone();
  