improve INVALID_NON_VIRTUAL_ANNOTATION message
improve INVALID_NON_VIRTUAL_ANNOTATION message

Change-Id: I47e5c5a86b3c46f03d3537592aa8c43bdb14c4a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122841
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Phil Quitslund <3faecbdd7b7be83f493f037bf1d597e6bb9d0848@google.com>

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