This splits visitors for (Tree)Visitor(1) into pure interfaces, mixins,
and a base class with the base implementation. This is a step towards
avoid having an accidental default implementation where a static error
would have been preferable.
This extract a ConstantReferenceVisitor(1) and its corresponding
DefaultMixin from the Visitor(1) interface.
TEST=existing
Change-Id: Ibc55bed9cff76581deaade91a10600c17fafc6dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/325704
Reviewed-by: Mayank Patke <fishythefish@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Johnni Winther
01f582df78