I am doing the angular2 tutorial at this address: https://angular.io/docs/ts/latest/tutorial/toh-pt3.html I have put the hero interface in a single
angular2
hero
restart ng serve
I had the same issue. In order to search for the error, I selected the error and accidentally did a CTRL+C (meaning to copy), which terminated ng serve. On restarting ng serve, the error disappeared :). Sometimes typos and fat fingers help ;-)