Ensure traceur System is exported not the global

If `global.System` is re-assigned, it is possible for `traceur.System` to reference the wrong `System`.

This fixes that by ensuring only the internal Traceur `System` is exported.
Can this be removed then?

Maybe you need to add `export {System} from './runtime/System';` before `export {ModuleStore} from '@traceur/src/runtime/ModuleStore';` due to side effects.