serialize parent attribute in child class

Hi,
I have this parent class:

```
@JsonSerializable()
class Parent extends Grandpa {
final Item item;

const Parent({
required id,
}) : super(
id: id,
);

factory Parent.fromJson(Map json) =>
_$ParentFromJson(json);

@override
Map toJson() => _$ParentToJson(this);
}
```

And this child class:
```
@JsonSerializable()
class Child extends Parent {
final List quantities;

const Child({
required id,
required item,
}) : super(
id: id,
item: item,
);

factory Child.fromJson(Map json) =>
_$ChildFromJson(json);

@override
Map toJson() => _$ChildToJson(this);
}
```

The script generates this code:
```
// GENERATED CODE - DO NOT MODIFY BY HAND

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

Child _$ChildFromJson(Map json) =>
Child(
id: json['id'],
item: json['item'], // <---- HERE I GET THE ERROR
);

Map _$ChildToJson(Child instance) =>
{
'id': instance.id,
'item': instance.item,
};
```

But I have an error cause `json['item']` is not a String, is a `Map` that should be converted with `Item.fromJson(json['item'] as Map)`