public final void containsKey(@Nullable Object key) {
    check("keySet()").that(checkNotNull(actual).keySet()).contains(key);
  }
@Test
  public void failMapContainsKeyWithNull() {
    ImmutableMap<String, String> actual = ImmutableMap.of("a", "A");
    expectFailureWhenTestingThat(actual).containsKey(null);
    assertFailureKeys("value of", "expected to contain", "but was", "map was");
    assertFailureValue("value of", "map.keySet()");
    assertFailureValue("expected to contain", "null");
    assertFailureValue("but was", "[a]");
  }