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