JsonConverter with import prefix

I have a `CustomJsonConverter` defined in a separate library which i want to import using a library prefix.

```dart
import 'library.dart' as converter;

@JsonSerializable()
class MyObject {
// ....

@converter.CustomJsonConverter()
final CustomObject value;
}
```

but the generated code uses `CustomJsonConverter.toJson` instead of including the `converter.` library prefix.

Not a big problem, since i could just remove the prefix, but it would be nice if this was supported.