Update status for front_end/test/mixin_export_test
Update status for front_end/test/mixin_export_test

Commit 48503c8720d758a61cc4ceb1281bdd81fbd92e13 introduces test case
pkg/front_end/test/mixin_export_test.dart that passes on d8, but doesn't
work in browsers.  An issue is created for that (dartbug.com/30576).
This CL changes the status for the tests on browsers.

R=johnniwinther@google.com

Review-Url: https://codereview.chromium.org/3008893002 .

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