_whenTransitionDone(element, itemCallback, done) {
    const id = onTransitionEnd(element, (evt) => {
      itemCallback();
      done(null, evt);
    });

    this._transitions.push(id);
  }
Listen for the transition end on an element and execute the itemCallback
when it finishes.
@param {Element} element Element to listen on.
@param {function} itemCallback Callback for the item.
@param {function} done Callback to notify `parallel` that this one is done.