Implement support for dynamic checks on parameters that are covariant-by-declaration due to inheritance

[eernstg]

Cf. dart-lang/language#925, consider the following program:

class A {}
class B extends A {}

class C {
  // The parameter `b` is not covariant-by-declaration as seen from here.
  void f(B b) {}
}

abstract class I {
  // If `I` is a superinterface of any class,
  // the parameter of its `f` is covariant-by-declaration.
  void f(covariant A a);
}

class D extends C implements I {} // OK.

void main() {
  I i = D();
  i.f(A()); // Dynamic type error.
}

The class D will behave in such a way that the actual argument to its f method has its type checked dynamically. In other words, when C.f is inherited by D, it is changed such that it's parameter is covariant-by-declaration. (This may be achieved by implicitly inducing a forwarding stub into D that calls super.f(...), but it could also be done by other means.)

This issue is concerned with the implementation of this feature. The tests added in https://dart-review.googlesource.com/c/sdk/+/208504 can be used as a guide.

This is not tied to a language version or a flag, because it only allows programs with errors to become running programs.

Subtasks:

• Specification update.
• Analyzer issue.
• CFE issue.
• dart2js issue.
• DDC issue.

[nshahan]

Can you comment on the priority of this issue? Are we expecting the implementation changes to be part of the next release?

[eernstg]

I sent out email to @devoncarew and some team members in order to find a suitable milestone.

[eernstg]

(Reopening, I think we should only close this when all subtasks are closed.)

[johnniwinther]

Yes. I accidentally used the wrong issue number in the fix for #47074

[devoncarew]

@eernstg - we may be complete here?

[eernstg]

Yep! Closing.

[devoncarew]

FYI, we bumped the analyzer issue out of the current stable milestone (and moved this meta issue as well).

[devoncarew]

I believe this is now complete.