System.register update

This includes the System.register update corresponding to the new format implemented into Traceur here - https://github.com/google/traceur-compiler/pull/1161.

All tests are passing!
Personally though, I don't like having a triple-lookup on the module object (property lookup -> getter -> property lookup). And I also don't like my idea of running `Object.defineProperty` repeatedly with a `value`. So I may just stick with allowing modules to be writable, as I wrote in the comment.