function createMethodDescriptor(responseDeSerializeFn) {
  return new MethodDescriptor(
      /* name= */ '', /* methodType= */ null, MockRequest, MockReply,
      (request) => [1, 2, 3], responseDeSerializeFn);
}
@param {function(string): !AllowedResponseType} responseDeSerializeFn
@return {!MethodDescriptor<!MockRequest, !AllowedResponseType>}