public final void containsKey(@Nullable Object key) {
    check("keySet()").that(checkNotNull(actual).keySet()).contains(key);
  }
@Test
  public void containsKeyNull() {
    Multimap<String, String> multimap = HashMultimap.create();
    multimap.put(null, "null");
    assertThat(multimap).containsKey(null);
  }