Last time I tried Dart (2020?), it was pretty easy to confuse the type system and get it to just abandon all hope of typing a fairly simple expression. I remember the early Dart presentations in which the developers very clearly stated that they didn't even try to make the Dart type system sound, because they felt that it would complicate the life of users.

Apparently, these days, Dart claims to be sound... except they seem to have redefined the meaning of type soundness along the way: https://dart.dev/guides/language/type-system#runtime-checks

Yes, sounds like you used it before Dart 2.0. As of Dart 2.0, the type system is sound in the same way that C#, Java, and Haskell have sound type systems, though a combination of mostly static checking with some runtime checks in a few places.