ES6: Added super test

This checks that:
* super() in the constructor binds to the superclass's constructor
* super.X() in the constructor binds to the superclass's X method
* super() in the X method binds to the superclass's X method
* super is bound statically, and remains bound even when a method is
extracted
Merge pull request #198 from webbedspace/super

ES6: Added super test