Meta-issue for discussion of the Dart 2.0 mixin proposal

[leafpetersen]

This issue is to provide a forum for community discussion and feedback related to the proposal to add explicit syntax for mixins in Dart 2.0.

An initial proposal is here: https://github.com/dart-lang/sdk/blob/master/docs/language/informal/mixin-declaration.md.

cc @munificent @eernstg @floitschG @lrhn

[rmacnak-google]

This change also requires breaking changes to dart:mirrors, which the proposal neglects.

[matanlurey]

How much real code do we have with mirrors + mixins being a critical user journey?

(Not trolling, I tried finding a single case and I couldn't)

[leafpetersen]

This is not planned for the release of 2.0.

[natebosch]

There is more discussion and context at dart-lang/language#7 which I think supersedes this proposal. I think we should close this one in favor of that one.

@rmacnak-google I added a comment there noting you had concerns about breaking changes in dart:mirrors