Should the use of both show and hide on an import be a syntax error?

In this example `hide` adds no meaning because if an identifier is not in the list of `show` it is not visible anyway:

```dart
import 'package:collection/collection.dart' show ListEquality hide MapEquality;

void main() {
ListEquality();
//MapEquality(); // undefined
}
```

Moreover an identifier can be in both `show` and `hide` resulting in it being hidden:

```dart
import 'package:collection/collection.dart' show ListEquality hide ListEquality; // ignore: unused_import

void main() {
//ListEquality(); // undefined
}
```

Both of these examples build and produce no linter issues.

Is there a valid case for using both `show` and `hide` on a single import? If yes, then there should be a lint that flags the meaningless cases like above. If no, should this be a syntax error? With Dart 3 allowed to introduce breaking changes, should this opportunity be used to add this syntax change?

Start with the two import examples in the issue and review the language-design discussion about combining show and hide. Determine whether the behavior should become a syntax error or a lint, and document the accepted rule and its effect on both examples as the completion criteria.

Written by the indexing model from the issue text.