Go-to-overriden-method

[jgiles]

We should provide an easy way to jump to an overriden method from the overriding method.

Though we could add this as a new type of request, it would be much better to do it in a client-independent manner. The easiest way to do this would probably be to catch go-to-definition calls in @override annotations, much like we want to within type strings (#28).

A definition request with a location inside an @override tag would return the definition location of the method up the prototype chain of the class.