[co19] Enable Metadata, Mixin and Overview tests
[co19] Enable Metadata, Mixin and Overview tests

Change-Id: I5a640e31eb50e17260386ef2aafe9864b54d8f8e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152642
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Commit-Queue: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@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