Better, friendlier, "more fun" no package 404 page

[kevmoo]

URL: https://pub.dartlang.org/packages/does_not_exist

When the target URL is clearly looking for a package, we could be better

1. Message that does_not_exist does not exist.
2. Link to search for does_not_exist

etc etc

[jakobr-google]

Agreed. We can have the search results (top 5 or 10) directly on the 404 page.

[mit-mit]

For inspiration: https://flutter.io/doesnexist