* Fix non-toplevel prototype assignment
binder was using the wrong node to lookup the containing class type for
prototype assignment, so it incorrectly put the prototype declaration on
the class' symbol.
This correction to the binder in turn required a change in
getJSClassType in the checker. It now has to look at the "prototype"
property for the prototype instead of looking on the class symbol's exports
(which makes no sense).
* Refactor per PR suggestion
