[munificent]

> only with code you wrote without real third-party libraries.

Your claim was true several years ago, but since then the entire ecosystem has migrated onto the new strict type system. It was a monumental amount of work, but, thanks to lots of effort from our users, we're there.

Aside from a small number of more-or-less dead packages, you should be able to use any third-party Dart packages you want while still using the new type system.