Update mirrors/syntax_error_test.dart for NNBD.
Update mirrors/syntax_error_test.dart for NNBD.

Change-Id: I7ca38af9d7651c9d11e50a54049416df9e00d44a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146699
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@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