Why can't we use classes in classes?

[stan-at-work]

This is a common and very helpful feature, in many languages.

This would reduce the number of files in a projects.
You can really scope your classes into one in other.

The you can have classes that have exacly the same name but live inside of 2 different parent classes.

This would solve the bad practice for having 2 classes with the same name in a project.

[lrhn]

Sounds like classes declared inside the static namespace of another class (or anything else that has a namespace).

It's probably not a problem to allow top-level type declarations to be declared as static members of other namespaces.

It can be a problem for extension declarations, because you'll need a way to avoid importing them, to avoid extension member conflicts.
So maybe just require those to be top: level.

It does mean they you'll need longer names to access the nested types, having to write fx Map.MapEntry.

The current solution to having multiple declarations with the same name is to use import prefixes.
That also requires more characters, like core.MapEntry, to overcome conflicts, but you only need those if you actually have a conflict in the library.

If you could open a static namespace, like, open Map show MapEntry;, then you'll have more flexibility in how to structure your lexical scopes.

There is an existing issues for nested static namespaces: dart-lang/language#2272