Add generator for AST visitors.

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 <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: fshcheglov