Make sure dartdoc for Flutter supports the new `mixin` syntax

[yjbanov]

I'm filing this issue proactively. I haven't gotten far enough to verify if this already works or not. However, practice shows that something like this is extremely unlikely to work by accident. If we know that this doesn't work yet, we need to make sure someone starts working on it asap.

If this is already done, feel free to just close the issue.

@JekCharlsonYu @jcollins-g @vsmenon

[jcollins-g]

This is already implemented in dart-lang/dartdoc#1765 and is blocked on an analyzer publish, which has some ecosystem impacts we're trying to work through.

It is tracked here: dart-lang/dartdoc#1752 and here: dart-lang/language#12

[yjbanov]

Cool. I'll keep this issue open. I changed the title to reflect that this is for Flutter specifically. Whatever analyzer/ecosystem issues that need to be resolved, should be resolved before we can consider this issue as fixed.

Unless the remaining issues are non-blocking?

[jcollins-g]

I would still prefer to track this in dart-lang/dartdoc#1752 as that issue is linked to the larger tracking issue, but if having this issue helps in some way you can leave this open.

[yjbanov]

Only because we've been tracking everything blocking the roll out of the new mixins into Flutter under Dart2.1 milestone here.

[dgrove]

dart-lang/dartdoc#1752 is closed. @jcollins-g can this be closed?

[jcollins-g]

I believe so -- some support definitely exists and there are no blockers I know of.