Analyzer does not recognize doc comment references of extension methods via extension target

[srawlins]

Given this code:

class C {
  int x = 1;
}

extension E on C {
  int y = 1;
}

/// [C.x]. [C.y]. [E.y].
void f () {}

Analyzer recognizes the comment references for [C.x] and [E.y], but not for [C.y]. I think it should recognize all three.

One argument is that C.x would not be a legitimate reference (or call or access or whatever) of the instance method x, because C.x would have to be a static member reference (or constructor reference). But we make an affordance in doc comment references for C.x since the reference (almost) cannot be ambiguous between a static member and an instance member ("almost" because a class can have a constructor named x and an instance member named x simultaneously).

So, we should make the same affordance for [C.y].

[srawlins]

Oops there is a more recent duplicate with more discussion. #56109