Implement non-function type alias support in Dartdoc

[leafpetersen]

This issue tracks support of the non-function type alias feature. See the enclosing meta-issue for details.

cc @franklinyow @mit-mit @jcollins-g @devoncarew

[leafpetersen]

@jcollins-g any updates here?

[jcollins-g]

dart-lang/dartdoc#2558 implemented most of this. There are some analyzer changes landed today that will make the rest of the planned support possible. Once that's published I can land the remaining changes.

[franklinyow]

Any updates?

[jcollins-g]

i am working on getting this published through the ecosystem as I write this.

[jcollins-g]

This is now published and propagated through Dart and Flutter.