@Test public void producingNullIsInvalid() {
    try {
      bus.register(new NullProducer());
    } catch (IllegalStateException expected) {
      // Expected.
    }
  }
Tests that events are distributed to any subscribers to their type or any
supertype, including interfaces and superclasses.

Also checks delivery ordering in such cases.
otto/src/test/java/com/squareup/otto/BusTest.java