From within a controller, you can access the router via this.get('target'). So this.get('target').send('goToNextStep') should work.
Like so:
App.Flow = Em.ObjectController.extend({
submit: function(){
// ...
this.get('target').send('gotoNextStep');
}
}
App.FlowRoute = Ember.Route.extend({
events: {
gotoNextStep: function(){
// ...
this.transitionTo(routeName);
}
}
}