Method createPlaceholderCallback

src/base-element.js:502–504

* Subclasses can override this method to create a dynamic placeholder * element and return it to be appended to the element. This will only * be called if the element doesn't already have a placeholder. * @return {?Element}

()

   * @return {?Element}
   */
  createPlaceholderCallback() {
    return null;
  }

  /**
   * Subclasses can override this method to opt-out of rendering the element