[vm/nnbd] Draft support for TypeRefs in CheckInstantiatedNullability
[vm/nnbd] Draft support for TypeRefs in CheckInstantiatedNullability

TypeRefs which are used to represent recursive types are now skipped
and removed when we need to change nullability of  type after
instantiation. This change assumes that we only need to change
nullability of a top-level type and not inner types.

Change-Id: I5e92493991f0e2a1ebeb16535ef9d507054017b4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133841
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk