Methods' this variable could be filled in even without calls

[marijnh]

When working on a library, there'll often be methods meant for external consumption that aren't called inside the library file itself. Their this will not get a type when they aren't called, and thus a lot of derived types also won't be found.

In methods living on a prototype, an instance of the prototype can be assigned to this. On objects that aren't prototypes, the object itself can be used.