Fix LeastUpperBoundTest tests for mixins.
Fix LeastUpperBoundTest tests for mixins.

When https://dart-review.googlesource.com/c/sdk/+/107444 landed,
it uncovered an issue in the mixin tests added in
https://dart-review.googlesource.com/c/sdk/+/110684

R=1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com, paulberry@google.com

Change-Id: If67d926f9762494693220ef39c8c8d5833d1c45e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/110744
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk