Go I'd consider, even if Google ditch it, the language still exists and is sufficiently entrenched to continue on. I wouldn't go near Flutter at all. Exactly the sort of thing that Google will lose interest in and stop backing leaving you with a product built on an un-supported platform.

Go is probably pretty safe, but even internally at Google Flutter is still sort of flying under the radar and nobody knows (or cares) about it too much. But if leadership decides to take a look at what the heck is going on in that corner of the organization... that would be a hard one to explain.