public StringSubject factValue(String key) {
    return doFactValue(key, null);
  }
@Test
  public void factValueFailNoValue() {
    Object unused = expectFailureWhenTestingThat(simpleFact("foo")).factValue("foo");
    assertFailureKeys(
        "expected to have a value",
        "for key",
        "but the key was present with no value",
        HOW_TO_TEST_KEYS_WITHOUT_VALUES.key);
    assertFailureValue("for key", "foo");
  }