Actually, Iran has been backing up Assad. ISIS is not good news for Iran.
As you know Iran is playing all sides....to win however....the idea is Assad will not be in control...and even iran now will let him fall.
I had thought that Iran is in fact funding Shiite militias in Iraq and Syria to fight ISIS. In any event, that Shia Sunni divide is definitely going to limit how powerful ISIS can get - ISIS is most likely going down hard if/when they try to force their way into the Shiite strongholds of Iraq. There is probably a reason the success they have had in Iraq and Syria has been in regions furthest away from Iran, i.e. Anbar.