`prefer_mixin` should not warn about non-super-mixin classes

[yjbanov]

Per dart-lang/language#32 many non-super-mixins will have to remain as classes indefinitely. Because the warnings from prefer_mixin show up at mixin application sites, and not at mixin declaration sites, it is not practical for it to warn about them. Mixin declarations are rare. Flutter declares them and makes developers apply them, but few other libraries and frameworks do. Therefore, the user who is applying the mixin rarely has the ability to change the mixin declaration.

Instead prefer_mixin should only warn about super-mixins, not regular mixins. Perhaps we should also use a different name for this lint. prefer_mixin_supermixin?

@leafpetersen @lrhn @eernstg @Hixie

[bwilkerson]

A separate lint might be a good choice. My concern is that it's a breaking change (in at least one sense) for a lint to start triggering for cases that it didn't used to trigger for. We could relax the lint now, but we're likely to want to restore it when we try to disallow mixing in classes.

[leafpetersen]

Instead prefer_mixin should only warn about super-mixins, not regular mixins.

Is the aim of this for managing the transition in flutter? That is, you want to be able to have a period of time where the old style super-mixins are supported but get linter warnings, before the old style super-mixins go away? Because otherwise you can just turn off the enableSuperMixins option, which should cause errors everywhere this lint would fire.

[yjbanov]

@leafpetersen I was thinking it could be used as an option for our users (I already turned off enableSuperMixins for Flutter's own code), but if we want to switch everyone to the same option, I'd be OK with that too. We'd be making a decision on behalf of our users, but I don't imagine our users defining their own mixins much, let alone super-mixins. So I think the effect is quite small.

[leafpetersen]

I was thinking it could be used as an option for our users (I already turned off enableSuperMixins for Flutter's own code)

It is definitely POR that enableSuperMixins will no longer be supported at all once Flutter has migrated. Based on my scraping of pub packages, I don't expect much (or any) user breakage due to this.

[yjbanov]

SGTM!