docs: public IterableSubject factKeys() {
      if (!(actual() instanceof ErrorWithFacts)) {
        failWithActual(simpleFact("expected a failure thrown by Truth's new failure API"));
        return ignoreCheck().that(ImmutableList.of());
      }
      ErrorWithFacts error = (ErrorWithFacts) actual();
      return check("factKeys()").that(getFactKeys(error));
    }