testNoListener() {
      dispatchEvent(eventTargets[0], EventType.A);
      assertNoOtherListenerIsCalled();
    }
Expando property used on "listener" function to record the number
of times it has been called the last time assertListenerIsCalled is
done. This allows us to verify that it has not been called more
times in assertNoOtherListenerIsCalled.