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!
Yes both are the proxy / Module object - it seems we are referring to the same thing just by different names.

Yes I am avoiding the performance penalty by not creating the proxy as a getter object.

Another alternative is to define directly to the module object:

```javascript
Object.defineProperty(moduleObj, 'export', {
  value: myValue,
  enumerable: true,
  configurable: true
});
```

The issue is the above gets done for each write, becoming expensive in circular scenarios.