`dart:mirrors` was deprecated on Dart for the web since 2.0.0. Both dart2js and DDC provided empty implementations of `dart:mirrors` that throw on every single API. Going forward both compilers will report a compile-time error when this library is imported.
Tools like webdev or flutter already show a compile-time error today, so this mostly affects users that work directly with the underlying tools (like invoking dart2js directly), if they happen to still have imports to `dart:mirrors` on their apps.
You can find more details at issue #42714
. Please comment on the bug if you have questions or concerns.