Add Dart 3 class modifiers to public classes

[nex3]

Pretty much all the classes we expose publicly in the Dart API should have some form of class modifier, but adding this would be a breaking change, so we should wait until 2.0.0.

[nex3]

I think it's fine to convert classes that have the @sealed annotation to sealed or final, but we shouldn't do so for un-annotated classes and we can't safely mark classes as base or interface.