function enqueueMarkup(parentID, markup, toIndex) {
  // NOTE: Null values reduce hidden classes.
  updateQueue.push({
    parentID: parentID,
    parentNode: null,
    type: ReactMultiChildUpdateTypes.INSERT_MARKUP,
    markupIndex: markupQueue.push(markup) - 1,
    textContent: null,
    fromIndex: null,
    toIndex: toIndex
  });
}
Generates a "mount image" for each of the supplied children. In the case
of `ReactDOMComponent`, a mount image is a string of markup.

@param {?object} nestedChildren Nested child maps.
@return {array} An array of mounted representations.
@internal