Remove check for declared library directive in test scripts.

Fix for http://youtrack.jetbrains.com/issue/WEB-10299 (Dart Plugin - Can't run test unless test is a library).

Tests do not need to declare a library so this check is not desirable.  

(Aside: I'm not entirely sure but it's possible that when this support was written this may in fact have been a requirement.  That said, it should be safe to remove now.)