Our tacit approval of Sarin use in the 1980s is not very relevant.
The fact (and it is a fact) that in the 1980s we told Saddam Hussein where Iranian troops were knowing that he would use the information to target nerve gas attacks is not a real argument about anything going on today.
It is one of those "throw everything and see what sticks" sort of arguments that appears to mean something at first glance, but doesn't add up to anything coherent.
(aka Facebook graphics arguments. A slogan, a picture, a factoid... seems to say something but makes no sense when parsed.)
Is the USA supposed to take a pro-Sarin stance because we have looked the other way on its use at some earlier point?
WTF? That could be said about slavery or using nuclear weapons or you-name-it.
Dropping bombs on people is a serious thing. When making an examination of the moral pros and cons of such a thing, whether it would be hypocritical is very, very far down the list of pressing moral concerns.
There are all sorts of reasons to not bomb Syria, but whatever Reagan did vis-a-vis Iraq is not one of those reasons. Reagan did not establish a pro-Sarin precedent that we are thus morally obliged to follow.
Makes. No. Sense.