[Class modifiers] Report a diagnostic when @reopen is placed on a class which hasn't reopened anything

[srawlins]

It'd be good to have a diagnostic for no-op @reopen annotations. Generally, it would report on any @reopen annotation which is attached to a class (or mixin?) which does not reopen anything. (@reopen annotations on things other than classes should already be reported as INVALID_ANNOTATION_TARGET.)

Examples:

// library lib1.dart
interface class Interface1 {}

// library lib2.dart
import 'lib1.dart';

interface class Interface2 {}

// WARNING: Does not re-open anything.
@reopen class C1 {}

// WARNING: Does not re-open anything, as it is illegal to extend Interface1 here.
@reopen class C3 extends Interface1 {}

CC @pq @bwilkerson

[pq]

/fyi @kallentu: if we captured information about induced modifiers in the element model, this is something we could neatly implement. Moreover, if we cached this data, we could do it quickly.

[pq]

Bumping this to the next milestone for future assessment. Landing @reopen at all is enough of a stretch IMO. (See #58976.)

[srawlins]

Mind if I take this one, Phil?

[pq]

Not at all. Go for it!