When saving an annotation to the server, the object that we actually send to
the server is:

1. `domainModel`, after being updated by `updateDomainModel()`, in the case
   of creating a new annotation, or

2. The temporary local variable `updatedDomainModel` in the case of editing
   an existing annotation

`vm.annotation` is never the object that gets sent to the server.

So pass `domainModel` or `updatedDomainModel` to `validate()`, not
`vm.annotation`.