code: static boolean isAdapterType(Element element, Elements elements, Types types) {
      TypeMirror typeAdapterType = types.getDeclaredType(
          elements.getTypeElement(TYPE_ADAPTER_CLASS_NAME),
          types.getWildcardType(null, null));
      return types.isAssignable(element.asType(), typeAdapterType);
    }
docs: Returns true if {@code element} is a {@code TypeAdapter} type.