_super() has historically not been particularly safe, being dependent

on how prototype chains were constructed. We've had the occasional

problem in the past that it didn't deal with well. And its assumptions

about what constitutes a parent prototype didn't mesh well with Spina.
Fortunately, the version of Backbone we're now using in Review Board 6

explicitly sets a __super__ attribute that points to the proper

parent. The upcoming Spina 2 includes this as well. This gives us a safe

way of using this method when objects are instances of Backbone or

Spina 2-wrapped classes.
This fixes upcoming issues with some mixin classes when used with the

upcoming Spina 2.

All unit tests pass.
Verified this worked correctly during in-progress work for Spina 2.