code: function lastChildElement (parent, callback) {
    for (let child = parent.lastElementChild; child; child = child.previousElementSibling) {
      if (callback(child)) {
        return child
      }
    }
    return null
  }
docs: Finds the last child element that satisfies the callback.
  @param {!Element} parent
  @param {function(!Element):boolean} callback
  @return {?Element}