* 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

      K key = entry.getKey();
      ImmutableList.Builder<V> values = (ImmutableList.Builder<V>) entry.getValue();
      ImmutableList<V> list =
          (valueComparator == null) ? values.build() : values.buildSorted(valueComparator);
      builder.put(key, list);
      size += list.size();
    }

    return new ImmutableListMultimap<>(builder.buildOrThrow(), size);
  }