Annotate members that override members from a superclass

[Hixie]

It would be great if on a page like the following, members that are defined in this class but override a member from a superclass were annotated as "override", the same way inherited members are annotated as "inherited".

https://domokit.github.io/docs/sky/rendering/RenderSizeObserver-class.html

[jcollins-g]

Given that @override annotations now show up in the feature list where 'inherited' does when those are used (and it seems to be increasingly Dart-standard to use them) I'm going to drop this a bit in priority.

To be clear, I'm interpreting this as adding an "override" to the feature list under the conditions @Hixie listed and using that to replace the @OverRide annotation.