fold-keys in type-check.arr

[sorawee]

In https://github.com/brownplt/pyret-lang/blob/master/src/arr/compiler/type-check.arr#L997

                variants-meet = cases(List<TypeMembers>) variant-type-fields:
                  | empty => empty
                  | link(first, rest) =>
                    cases(List<TypeMembers>) rest:
                      | empty => first
                      | link(_, _) =>
                        rest.foldr(meet-fields(_, _, l, context), first)
                    end
                end
                extended-shared-field-types = variants-meet.fold-keys(lam(key, extended-shared-field-types):
                  extended-shared-field-types.set(key, variants-meet.get-value(key))
                end, initial-shared-field-types)

But this is clearly wrong -- the first branch of the case statement says variants-meet is a List, so it can't variants-meet.fold-keys

[sorawee]

Minimal program:

data False:
end