0a0bae399f
Instead of using the coutner of type parameter substitution, return null when instantiation doesn't change the type, similarly to how ReplacementVisitor works. This is a follow up to https://dart-review.googlesource.com/c/sdk/+/312264 Change-Id: I839c624e4f161d24f295be4f38574ed607f98c95 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/328880 Reviewed-by: Johnni Winther <johnniwinther@google.com> Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>