Aha, this is the problem.  Due to various reasons (see http://lists.ocaml.org/pipermail/opam-devel/2014-April/000507.html if interested), you can't have constraints in optional dependencies.  Instead, add the opposite as a conflict line.  So in this case remove the constraint on `async_extra` and add:

```
conflicts: [ "async_extra" {>"111.11.00"} ]
```