UnavailableType does not provide the correct assembly #430

When an assembly dependency is not available we end up with UnavailableType instances for references to types from this assembly, which is perfectly fine. However I would expect the UnavailableType.Assembly property to return information of the (missing) assembly containing the unavailable type instead of the assembly that references this type. Currently we could even have 2 different instances of UnavailableType (with a different Assembly property) for the same external type if it's referenced from two different assemblies.