allow non-concrete serialized types provided all implementations are serializable

[Andersmholmgren]

See google/built_value.dart#22

I was forced to create a custom serialiser (https://github.com/Andersmholmgren/vcore/blob/master/lib/src/model/model.dart#L118) because I have properties using a base type.

abstract class Classifier implements NamedElement {}

abstract class GenericType
    implements Built<GenericType, GenericTypeBuilder>, Classifier {
  Classifier get base;
  BuiltMap<TypeParameter, Classifier> get genericTypeValues;
...
}

Properties like base and genericTypeValues need to use the abstract type Classifier. The Json serialiser generator doesn't cope with this so I needed to create a custom serialiser

Serializers serializers =
    (_$serializers.toBuilder()..add(_classifierSerializer)).build();

//final x = new SerializersBuilder()..;
final _ClassifierSerializer _classifierSerializer = new _ClassifierSerializer();

class _ClassifierSerializer implements StructuredSerializer<Classifier> {
  @override
  Classifier deserialize(Serializers serializers, Iterable serialized,
      {FullType specifiedType: FullType.unspecified}) {
    return serializers.deserialize(serialized);
  }

  @override
  Iterable serialize(Serializers serializers, Classifier object,
      {FullType specifiedType: FullType.unspecified}) {
    return object != null ? serializers.serialize(object) : [];
  }

  @override
  Iterable<Type> get types => [Classifier];

  @override
  String get wireName => 'Classifier';
}

[davidmorgan]

Sorry, I thought I'd replied to this. Yes, this should be supported. I'll take a look.

[davidmorgan]

This should do it

#26

As before, happy if you want to review, or I'll find someone here. Thanks!