Get items from the SearchApiEvents::INDEXING_ITEMS event

Problem/Motivation

After dispatching SearchApiEvents::INDEXING_ITEMS event (Index::indexSpecificItems) the set items in the event are lost.

Proposed resolution

    $event = new IndexingItemsEvent($this, $items);
    \Drupal::getContainer()->get('event_dispatcher')
      ->dispatch(SearchApiEvents::INDEXING_ITEMS, $event);
    $items = $event->getItems();