Skip enum_mirror_test under dart2dart.

TBR=hausner@google.com
BUG=dart:11511

Review URL: https://codereview.chromium.org//748483004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41944 260f80e4-7a28-3924-810f-c04153c831b5