Makes for an interesting question. It would certainly be unusual to use a brand-new untested weapon sent on a combat mission so soon. On the other hand, the IAF

Makes for an interesting question. It would certainly be unusual to use a brand-new untested weapon sent on a combat mission so soon. On the other hand, the IAF has been known for sometimes doing bold, risky, and unusual things, and hitting the Syrians with brand-new F-35s would have a lot of value in reminding the Assad regime not to mess with them. On the other hand, how many people can tell the difference between a F-35 and a F-16 streaking by at 500mph, dropping a bunch of bombs with no warning whatsoever? It also wouldn't be too surprising to see some unfortunate Syrian soldiers tell their superiors that they were hit by F-35s when they actually have no idea what hit them. Makes them look less bad that they failed to stop an attack by a super-high-tech jet, and probably good for surviving it. It would even fit for the Israelis to subtly encourage any such rumors, if they could.