code: static boolean isSingletonAdapter(
        Elements elements, Types types, TypeElement element, TypeMirror adaptedType) {
      TypeElement typeAdapterElement = elements.getTypeElement(TYPE_ADAPTER_CLASS_NAME);
      DeclaredType typeAdapterType = types.getDeclaredType(typeAdapterElement, adaptedType);
      return isSingleton(types, element, typeAdapterType);
    }
docs: A singleton is defined by a class with a public static final field named "INSTANCE"
  with a type assignable from a {@code TypeAdapter} of {@code adaptedType}.