Map with enum keys is serialized as List, but deserialized as Map

Just had some fun figuring out why this fails:

```dart
import 'package:equatable/equatable.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:json_annotation/json_annotation.dart';

part 'enum.g.dart';

enum A { a, b }

@JsonSerializable(anyMap: true, explicitToJson: true)
class B extends Equatable {
final Map map;

Iterable get values => map.values;

const B([Map? values]) : map = values ?? const {A.a: 1, A.b: 2};

B update(A a, int value) {
return B(Map.from(map)..[a] = value);
}

factory B.fromJson(Map json) {
return _$BFromJson(json);
}

Map toJson() => _$BToJson(this);

@override
List get props => map.values.toList();
}

void main() {
test('description', () {
final instance = const B().update(A.a, 3);
expect(B.fromJson(instance.toJson()), equals(instance));
});
}

```

Having a getter with the same name as a constructor parameter leads to _that_ one's values being serialized rather than the actual field. It would be great to

- ideally have this not compile without explicitly making it,
- generate a warning during build runner,
- maybe reflect this somewhere on the pub page? I might have missed it but at least the word "getter" is not present.