add matchTarget to event forwarded by on()

See https://bugs.dojotoolkit.org/ticket/18355
This all seems a bit much. Why not just assign `event.matchTarget = eventTarget;` after https://github.com/ben8p/dojo/blob/18355/on.js#L267? That would prevent adding 7 lines of code and also wouldn't assign `event.matchTarget` unless it's a delegated event.