enable banning ES5 features (encourage "let" over "var", etc) using the conformance framework

The conformance framework currently runs after type checking, and type checking currently requires rewriting to ES5 and makes it impossible to use the conformance framework to encourage ES6 features.

One option requires waiting until the type checkers support ES6 natively

Another option would be to add a option to run some conformance before type checking (and before ES6 translation specifically).