Fix literals for polymorphic types

[timbod7]

currently commented out in test4:

// FIXME: Should work, but gives error:
//        Invalid override of field v8a of S: literals not allows for parameterised fields
//
//    Map<String,Int32> v8a = [
//      { "v1" : "X", "v2" : 1 },
//      { "v1" : "Y", "v2" : 2 }
//      ];
};