private List<FieldSpec> properties() {
      return propertyMirrors.stream()
          .map(
              propertyMirror ->
                  FieldSpec.builder(
                          TypeName.get(
                              serializersMap
                                  .get(MoreTypes.equivalence().wrap(propertyMirror.getType()))
                                  .proxyFieldType()),
                          propertyMirror.getName(),
                          Modifier.PRIVATE)
                      .build())
          .collect(toImmutableList());
    }
Maps each AutoValue property to a serializable type.
value/src/main/java/com/google/auto/value/extension/serializable/processor/SerializableAutoValueExtension.java