Respect Dart 3 class modifiers when treating an interface "required"

[devmil]

Is your feature request related to a problem? Please describe.
Dart 3 introduces sealed classes.
Classes that are marked as sealed are not meant to be implemented on the consumer side of a package and therefore shouldn't be treated as required interfaces.
Additionally there are more new modifiers for classes that exclude them from being "required" (https://github.com/dart-lang/language/blob/master/accepted/future-releases/class-modifiers/feature-specification.md):

• final: prevents extension from outside

Describe the solution you'd like
If a class is marked as sealed (or can't be extended by the consumer because of any of the new modifiers) then it should not be treated as required interface when passed into the API.