@Override
  public void addUnique(String text) {
    if (this.seen.add(text)) {
      this.messages.add(new Message(text, system2.now()));
    }
  }
@Test(expected = IllegalArgumentException.class)
  public void addUnique_fails_with_IAE_when_message_is_empty() {
    underTest.addUnique("");
  }