Migrate mixins to be mixin declarations about file.dart HOT 8 CLOSED

cc @itsjustkevin I am not sure if there is a good meta issue to attach this to for tracking

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@devoncarew do you have the correct permissions to tag this somehow?

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For sure, I can apply labels here. But probably good to also add as an item in a meta-issue in the sdk repo?

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We have a "Dart 3" project that the other issues are linked to, but I don't know that we can add issues from other organizations to if.

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Probably useful to have a proxy issue in dart-lang/sdk which mentions this one (I think you're right that projects can't span github orgs).

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Note that this package is also used by flutter/flutter and so this may cause issues there too. Although as long as this package stays on an old language version it might still be OK.

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Once this get resolved, we can remove a couple of ignores in the flutter_tool added here: flutter/flutter#123159.

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I made a PR enabling Dart 3. It was fairly easy. I don't know who is the code owner for this repo, but whoever is, feel free to review when you have time.

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