public final void doesNotContainKey(@Nullable Object key) {
    check("keySet()").that(checkNotNull(actual).keySet()).doesNotContain(key);
  }
@Test
  public void doesNotContainKey() {
    ImmutableMultimap<String, String> multimap = ImmutableMultimap.of("kurt", "kluever");
    assertThat(multimap).doesNotContainKey("daniel");
    assertThat(multimap).doesNotContainKey(null);
  }