message: feat(devtools): warn against old devtools version
  See
diff: @@ -72,6 +72,12 @@ export function addDevtools(app: App, router: Router, matcher: RouterMatcher) {
  app,
  },
  api => {
  + if (typeof api.now !== 'function') {
  + console.warn(
  + '[Vue Router]: You seem to be using an outdated version of Vue Devtools. Are you still using the Beta release instead of the stable one? You can find the links at https://devtools.vuejs.org/guide/installation.html.'
  + )
  + }
  +
  // display state added by the router
  api.on.inspectComponent((payload, ctx) => {
  if (payload.instanceData) {
  