Remove MirrorsUsed and Comment from dart:mirrors before Dart 2.0 release

[rmacnak-google]

Dart2JS no longer implements mirrors; as such its cruft should be removed from mirrors.

• MirrorsUsed was added only for dart2js to inform its tree shaker.
• Comment was only used internally by dart2js, and it was never possible to get an instance at runtime.

[sigmundch]

For a transitional period, we still allow importing dart:mirrors from dart2js, but we fail at runtime. That means that using the @MirrorsUsed annotation is ignored. If we remove it before we make importing mirrors a compile time error, we risk introducing an unintended breaking change.

We should time these changes to happen at once.

[kevmoo]

At a minimum, we should mark MirrorsUsed as deprecated ASAP IMHO

[anders-sandholm]

Fixed (MirrosUsed marked as deprecated) in cdc520f

[rmacnak-google]

The issue is for removal, not deprecation.

[Sunbreak]

MirrorUsed is still there. We would keep it or not?

[lrhn]

We still want to get rid of it, but there were other higher-priority issues in the Dart 2.0 release, so it didn't make it there.

It doesn't do anything anymore, so we should be able to remove it at any time (preferably along with some other deprecated declarations, in one fell breaking change).

[Sunbreak]

Is there any alternatives for escaping from tree shaking?

https://github.com/google/reflectable.dart seams to be stale as well

[eernstg]

The package reflectable hasn't been abandoned, I'm keeping it alive in my spare time.

It hasn't been updated to handle types with nullability (so if you have a TypeMirror you can't ask whether the type is nullable or not), but it works fine with programs using the current language, in partially or fully null-safe programs. It will be extended to expose nullability when I can find some time, but you may well be able to use it also without that feature. Of course, dart:mirrors also do not expose nullability in their TypeMirror, yet.

Ah, by the way, you might be able to test nullability using TypeMirror.isSubtypeOf to check whether it is a subtype of Object and whether Null is a subtype of it: If both are true then it is a legacy type, if the former is true and the latter is false then it is a non-nullable type, and if the former is false and the latter is true then it is a nullable type.

So you could take a look at reflectable! ;-)

[Sunbreak]

Thanks a lot. I'll try it and post the result later

[mit-mit]

Fixing in https://dart-review.googlesource.com/c/sdk/+/248003