toJson() convert

[aliyazdi75]

Hi, thanks for your plugin.
Since serializers.serializeWith returns Object type, it would be very nice to add as Map<String, dynamic> at the end of this return serializers.serializeWith(Property.serializer, this) as Map<String, dynamic>;

[ndcollins]

Or alternatively to simply use the serializers.toJson and serializers.fromJson methods

[GiancarloCode]

Hello, I have not used built_value since version 7, the weekend I will update myself, to update the snippets, in the same way if you want to make a pull request or leave a draft of how you want them to be, it would be of great help :)