static getFirstDescendantWithClassName(element, className) {
    // TODO: This can be replaced by shaka.util.Dom.getElementByClassName

    const descendant = shaka.ui.Utils.getDescendantIfExists(element, className);
    goog.asserts.assert(descendant != null, 'Should not be null!');

    return descendant;
  }
@param {!HTMLElement} element
@param {string} className
@return {!HTMLElement}