20c8756bc8
Update abstract classes into proper mixins since that how they are used. Change-Id: I8ce43f873a2aacab48d859de633f531f091e58e6 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/325143 Commit-Queue: Nicholas Shahan <nshahan@google.com> Reviewed-by: Mayank Patke <fishythefish@google.com>