This makes both compilers report a compile-time error when importing dart:mirrors.
Change-Id: I23360b87ee31fba36fe7c111cee1987aa2d50638
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154622
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>