Analyzer doesn't error on legacy libraries #50694
Description
Repro steps with current main:
$ dart create testapp
$ cd testapp
$ nano lib/testapp.dart
Add // @dart=2.9 in that file
$ dart analyze .
Analyzing .... 1.2s
info • bin/app2.dart:1:8 • The library 'package:app2/app2.dart' is legacy, and shouldn't be imported into a null safe library. Try
migrating the imported library. • import_of_legacy_library_into_null_safe
info • test/app2_test.dart:1:8 • The library 'package:app2/app2.dart' is legacy, and shouldn't be imported into a null safe
library. Try migrating the imported library. • import_of_legacy_library_into_null_safe
2 issues found.
Expected result: an error
Actual result: two infos