public final void doesNotContainKey(@Nullable Object key) {
    check("keySet()").that(checkNotNull(actual).keySet()).doesNotContain(key);
  }
@Test
  public void doesNotContainNullKeyFailure() {
    Multimap<String, String> multimap = HashMultimap.create();
    multimap.put(null, "null");
    expectFailureWhenTestingThat(multimap).doesNotContainKey(null);
    assertFailureKeys("value of", "expected not to contain", "but was", "multimap was");
    assertFailureValue("value of", "multimap.keySet()");
    assertFailureValue("expected not to contain", "null");
    assertFailureValue("but was", "[null]");
  }