Cast does not parse generic parameters

[BSteffaniak]

let packageFile = new File("#{System.workingDirectory}/flat.json")
let map = new JsonDeserializer().parse(packageFile)
let package = new Package((HashMap)map)

This works, but this fails:

let packageFile = new File("#{System.workingDirectory}/flat.json")
let map = new JsonDeserializer().parse(packageFile)
let package = new Package((HashMap<String, Object>)map)