Do it the right-ish way, with performEvent instead of unsafePerformIO

The key insight here is that the type of _frontend_body does not bring enough
constraints along to use performEvent, even though the execution
context (native, anyway) can satisfy those constraints.

Splitting the code out into a separate function made the compiler error much
more understandable for me and it just became a matter of adding the necessary
typeclass constraints.

It remains to be seen whether this approach actually gives you something that
will work on Android, though.