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!
I'm not sure I follow. The module object returned by `System.import('foo')` is not one that is writable, but it does have live bindings. Because it is the true module object, it is getting updated naturally.

The same principle applies to an imported module object. It has live bindings on the object, but those bindings are not writable.

Thus there's no need for getters or setters at all in this process.