[Class modifiers] Language feature docs

[MaryaBelanger]

Summary

Class modifiers determine how a class can be implemented, extended, or mixed in relative to the library it's defined in. Invariants like this are up to the library author; they can make types easier to understand and use.

Evaluation

1. Class modifiers will be a stand-alone page under "Language > Classes & objects" in the side nav

   There may also be coverage, or at least references to the core coverage, somewhere in the "Libraries" content.

2. The table of combinations:impacts is NOT necessary

   • just the use cases, start with important ones

3. Mixin docs need to be updated: classes can't be used as a mixin by default anymore, must explicitly use the modifier mixin class

• Class modifiers content needs to come after existing mixin content, because all the modifiers apply to mixins and classes.
• See Mixin restrictions section of spec for changes to mixins around the allowance of constructor declaration (currently can't declare a constructor, restrictions loosened)

5. Potentially, for each new class modifier/class modifiers combination, briefly mention the use cases under "Motivation" in the spec to give users an idea of when to use this.

   e.g. "The final modifier` ... disallowing implementation and extension of a type makes it entirely safe to add new members to"

   Another way to organize this information would be the inverse, i.e. by motivation: "The modifiers disallow implementing or extending, which is good for.... "

6. Make sure to add the page link to related diagnostics e.g. + implicit_reopen linter#4101

7. See [Dart 3] Document general use of class modifier by Dart core libraries in Library Tour #4730

• will open another PR to address this since the first one is pretty large

8. (maybe) add something to development or package content about how modifiers are for package maintainers / developers who want to limit the usage capabilities for others using their package

• skip

9. "extend a class" page and "implicit interfaces" section need to point to class modifiers page ("class modifiers restrict how classes can be extended/implemented")

• think I'll skip this for now

10. (maybe) move "abstract classes" section to class modifiers page?

• need to find and fix links to it across the site, add redirect

12. add mention + link to class modifiers in intro paragraph of main "Classes" page

13. Not sure if anything needs to change in "nosuchmethod" section based on this note:

    If you have a class that is marked base or final, that means that you are ensured when an instance of that class is passed into your library that any access of a private member on it will never throw a nosuchmethod error

14. Change link for interface on 'Keywords' page #4662

Resources

• https://github.com/dart-lang/language/blob/master/accepted/future-releases/class-modifiers/feature-specification.md
• https://dart.dev/guides/language/language-tour#adding-features-to-a-class-mixins