I have the same issue. After upgrade to Ionic 2 - RC2 I am getting:
[22:35:05] copy: Error copying "/Users/tyrion/devel/saveup-front/src/service-worker.js" to
"/Users/tyrion/devel/saveup-front/www/service-worker.js": File not found
Ok I fixed this for now, by just copying the service worker file from the conference app by Ionic team over to my project. So far working.