Supermixin support is not currently planned for the initial release of
Dart 2.0, so it seems unwise for front end and kernel to enable it.
Kernel didn't actually contain any usages of supermixin support; front
end contained one usage in a test.
Change-Id: I2b225f8d6c5f0ea32cb60d7636e043c89439548c
Reviewed-on: https://dart-review.googlesource.com/3020
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
