Router Migration Guide links give 404

[benhalverson]

The link to the router example does not exist :disappointed: starting from https://angulardart.dev/note/router/migration

https://github.com/dart-lang/angular/tree/master/angular_router/example gives a 404

[chalin]

Engineering Notes are no longer hosted on angulardart.dev. So I'm transferring this issue to the Angular repo where the note sources are found. In particular this issue now relates to the "Router Migration Guide" link in https://github.com/dart-lang/angular/blob/master/doc/migrating-to-v5.md

[chalin]

cc @kwalrath @legalcodes @johnpryan