In https://github.com/flutter/engine/pull/45870/files/d9735741e0980df0020e23f48a5cff904b923970#r1327787268:

@chinmaygarde:

The locking without static analysis makes me nervous. I suspect we didn't use the IPLR_GUARDED_BY and friends because we don't have static thread safety analysis around condition variable. This should be easy enough to add support for. Can we file a followup bug for it?

So, this is that bug!

I also mentioned I'd personally prefer to just use absl if possible (#134741), but that's not necessarily a blocker.