'We interpose on EventMixin.addListener to check if the eventType is
in our promise list. If so, trigger the handler for all previously
triggered events.'

def addListener(self, eventType, handler, once=False, weak=False, priority=None, byName=False):

    if (eventType in self._event_promises):
        self._event_promises[eventType](handler)
    return EventMixin.addListener(self, eventType, handler, once=once, weak=weak, priority=priority, byName=byName)
