Add implementation classes for pattern AST nodes
Add implementation classes for pattern AST nodes

Visitor support is underway.

Change-Id: If4e54515650d21a202635973e2ce63b686bb98f5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/258400
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@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