Classes and mixins section of Design chapter of Effective Dart should be rewritten accounting class modifiers feature of Dart 3.0

[ViktorVoloshko]

Page URL

https://dart.dev/effective-dart/design#classes-and-mixins

Page source

https://github.com/dart-lang/site-www/tree/main/src/content/effective-dart/design.md

Describe the problem

Guidelines

• DO document if your class supports being extended
• DO document if your class supports being used as an interface
  Don't take into account class modifiers: https://dart.dev/language/modifier-reference
  This also affects other guidelines
• AVOID extending a class that isn't intended to be subclassed
• AVOID implementing a class that isn't intended to be an interface

Since Dart 3.0 limiting user's ability to extend/implement a class with class modifiers is a language feature, no need to be explicit in docs.

Expected fix

Change guidelines for API maintainers so that if a class not meant to be extended/implemented, they should use according class modifiers.

Remove guidelines for users since those are obsolete if API makes use of class modifiers.

Additional context

No response

I would like to fix this problem.

• I will try and fix this problem on dart.dev.

[MiniPiku]

The issue have been fixed
kindly review the PR and suggest any necessary changes if needed :
#6449

Thank you