Method mountCallback

src/base-element.js:563–563

* Load heavy elements, perform expensive operations, add global * listeners/observers, etc. The mount and unmount can be called multiple * times for resource management. The unmount should reverse the changes * made by the mount. See `unmountCallback` for more info. * * If this callba

(opt_abortSignal)

   * @return {?Promise|undefined}
   */
  mountCallback(opt_abortSignal) {}

  /**
   * Unload heavy elements, remove global listeners, etc.