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For now only includes `SimpleAstVisitor` `ThrowingAstVisitor`, `RecursiveAstVisitor`, and `UnifyingAstVisitor`, but I will add more. Change-Id: I71d80c56eb9fa6a66e2a2050e987f87a70a249f8 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442380 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Reviewed-by: Konstantin Shcheglov <scheglov@google.com> Commit-Queue: Konstantin Shcheglov <scheglov@google.com>