Switch-cases completion is missing after Dart 3.0

## Steps to Reproduce
1. Set the `sdk` constraint to `>=3.0.0` in `pubspec.yaml` and run pub get.
2. Try the below code.
```dart
enum MyEnum { foo, bar }

void f(MyEnum e) {
switch (e) {}
}
```


## Version info

```console
[√] Flutter (Channel stable, 3.10.5, on Microsoft Windows [Version 10.0.22621.1848], locale en-US)
• Flutter version 3.10.5 on channel stable at X:\SDK\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 796c8ef792 (3 weeks ago), 2023-06-13 15:51:02 -0700
• Engine revision 45f6e00911
• Dart version 3.0.5
• DevTools version 2.23.1
• Pub download mirror https://pub.flutter-io.cn
• Flutter download mirror https://storage.flutter-io.cn
```