public final void containsKey(@Nullable Object key) {
    check("keySet()").that(checkNotNull(actual).keySet()).contains(key);
  }
@Test
  public void containsKey_failsWithNullStringAndNull() {
    Map<String, String> actual = Maps.newHashMap();
    actual.put("null", "value1");

    expectFailureWhenTestingThat(actual).containsKey(null);
    assertFailureKeys(
        "value of",
        "expected to contain",
        "an instance of",
        "but did not",
        "though it did contain",
        "full contents",
        "map was");
    assertFailureValue("value of", "map.keySet()");
    assertFailureValue("expected to contain", "null");
  }