Raise a warning when an ES6 class method doesn't call super

When an ES6 class extends another and overrides a method, it's pretty easy for the user to forget to call `super`. Unfortunately, this is a check that is beyond the standard JS linting tools as it requires whole world type information.

I'm proposing adding a new diagnostic group which issues a warning when it sees this. Users can suppress the warning if the omission was intentional.