IllegalArgumentException.class,
        () -> filtered.putAll(ImmutableMap.of("c", 3, "zzz", 4, "b", 5)));

    assertEquals(ImmutableMap.of("a", 1, "b", 2), filtered);
  }

  public void testFilteredKeysFilteredReflectsBackingChanges() {
    Map<String, Integer> unfiltered = createUnfiltered();
    Map<String, Integer> filtered = Maps.filterKeys(unfiltered, NOT_LENGTH_3);
    unfiltered.put("two", 2);
    unfiltered.put("three", 3);