new lint: avoid_overriding_widget_equals

[pq]

Follow-up from flutter/flutter#41220 (comment) (and flutter/flutter#1220).

Intent: disallow overriding == and hashCode in Widget subclasses.

Discussion: consider a general solution such as a @final annotation (or extending @sealed in package:meta).

/cc @bwilkerson @Hixie

[pq]

The more I think about this, the more inclined I am to tackle the case generally w/ an annotation in meta (dart-lang/sdk#34378) and general purpose enforcement in analyzer. Would love to hear other folk's thoughts...

[Hixie]

I definitely prefer general lints that are triggered by annotations (a la @immutable) than Flutter-specific things.

[pq]

We went ahead and took a general approach and implemented @nonVirtual in package:meta:

To take advantage Widget should get annotated. (And possibly elsewhere too.)