System.register circular dependency form

I think this actually just about does it for #953.

The remaining change is to do variable rewriting so that all occurrences of `dep` become `$__deps[2].dep`. Do we have scope analysis in Traceur currently to allow this? Quite a big hurdle otherwise, but live bindings can always wait.

There is one test failing, and that is the test for integrating this into the existing Traceur loader. I couldn't work out what is going on with that, but will have another attempt soon.