Should we provide a dynamically checked function type?

**Edited** Editing this to re-point this to the more general problem of having a type for dynamically checked functions, without being specific as to the mechanism (using `Never` as the marker being one particular strategy).

The type `int Function(Never)` is useful only as a top type for unary functions. It could be useful to allow calls to this to be done using dynamic runtime checks, since in practice all deliberate uses of such function types currently require going through a dynamic call.

A concern is the possibility of inadvertent uses - are there scenarios this might arise accidentally through inference?