* Returns a newly-created {@code ImmutableList} based on the contents of the {@code Builder},
     * sorted according to the specified comparator.
     */
    @SuppressWarnings("unchecked")
    ImmutableList<E> buildSorted(Comparator<? super E> comparator) {
      // Currently only used by ImmutableListMultimap.Builder.orderValuesBy.
      // In particular, this implies that the comparator can never get "removed," so this can't