GHCJS has to jump through hoops to make annotations work and to get the necessary information for linking a TH splice. The main reason is that hscCompileCoreExprHook does not expose the TcM environment.

Easiest fix would be to add

hscCompileCoreExprTcMHook :: HscEnv -> SrcSpan -> CoreExpr -> TcM HValue

to Hooks, but it'd be better to hook runMeta instead. Unfortunately runMeta is polymorphic, so it could be more tricky. I'm going to see if I can hook runMeta in a reasonable way, and if I run out of time for 7.10.1, send a patch with hscCompileCoreExprTcMHook instead.