support the new super mixins

[devoncarew]

The overall tracking issue is here: dart-lang/language#12.

This is the dartdoc specific issue for supporting the new super mixins feature.

[jcollins-g]

Marking as blocked until at least some level of support in analyzer (dart-lang/sdk#34164) is available.

[jcollins-g]

Still blocked for completion (will require a version of build to be published that supports the new analyzer as well). However, I have wired this up enough now to know that the new analyzer support works well enough to treat the new mixin declarations as ordinary classes.

I think the right thing to do here is to treat these similarly to how we treat enums; as a special type of class with different labeling.

[jcollins-g]

Landing this, and a dartdoc publish is blocked on this: dart-lang/build#1873 and a new analyzer publish which should happen in the next day or so.

[jcollins-g]

As pointed out in dart-lang/build#1873, there is a workaround available for us to publish without new build versions, so we're really only blocked for certain on a new analyzer publish.