private static final ImmutableSortedMap<Integer, Integer> SORTED_SINGLETON =
      ImmutableSortedMap.of(1, 2);

  public void testMapDifferenceOfSortedMapIsSorted() {
    Map<Integer, Integer> map = SORTED_SINGLETON;
    MapDifference<Integer, Integer> difference = Maps.difference(map, EMPTY);
    assertTrue(difference instanceof SortedMapDifference);
  }