function enqueueInsertMarkup(parentID, markup, toIndex) {
  // NOTE: Null values reduce hidden classes.
  updateQueue.push({
    parentID: parentID,
    parentNode: null,
    type: ReactMultiChildUpdateTypes.INSERT_MARKUP,
    markupIndex: markupQueue.push(markup) - 1,
    content: null,
    fromIndex: null,
    toIndex: toIndex
  });
}
Enqueues markup to be rendered and inserted at a supplied index.

@param {string} parentID ID of the parent component.
@param {string} markup Markup that renders into an element.
@param {number} toIndex Destination index.
@private