I think snack bars should be a feature of a Scaffold. They probably shouldn't use routes at all.

According to the specs -- https://www.google.com/design/spec/components/snackbars-toasts.html#snackbars-toasts-specs -- they should not time out while another route (e.g. a dialog) is over them, they should reset their timeout when their parent route resumes being the current route, they should be displayed consecutively in time, etc.