Representation of redirecting factory bodies in Kernel AST

[peter-ahe-google]

To support modular compilation, we need a a way to represent redirecting factories, including the unresolved kind. I've mocked it up in Fasta in RedirectingFactoryBody.

See also #29169.

[mkustermann]

When doing the kernel2kernel constants transformation I've run into this issue, since the redirecting factory constructor's body has a StaticGet(target-constructor) and my code treated all static tear-offs as constants (and there's no such thing as a constructor tear-off yet).

Also package:kernel/verifier.dart barfs on a fasta generated dill file (we use this verifier e.g. in pkg/kernel/bin/transform.dart).

If we need to preserve the information in modular compilation outputs, it seems to make sense to have a construct in Kernel IR to represent it (is it e.g. needed for dart:mirrors?).

If we decide to do it differently, e.g. by lowering factory constructors to have a body with a tailcall and a hint to not show it in stack frames, then we might not need it.

Would be wonderful if we could come up with a solution to fix this issue.

[kmillikin]

@stefantsov Could you take a look at implementing this when you get a chance?

[chloestefantsova]

@kmillikin Sure. I plan to start working on it today.

[askeksa-google]

The representation is there. It is currently not emitted correctly, which is tracked by #28421 and #28424