test returning too-large int from native

@marco-c: I noticed this while working on TextEditor, and I'm unsure what exactly the problem is, but it seems like a problem. If a native returns a numeric value outside the range of its int return type, I would expect the VM to either throw an exception or convert the value to a valid int (via `val | 0`, as I did in #265). But it doesn't do either of those things. Instead, it returns a value that stringifies to the original numeric value, although I'm not quite sure what type it actually is.