ServiceExtensionResponse.error(int errorCode, String errorDetail)

Creates an error response to a service protocol extension RPC.

Requires errorCode to be invalidParams or between extensionErrorMin and extensionErrorMax. Requires errorDetail to be a JSON object encoded as a string. When forming the JSON-RPC message errorDetail will be inlined directly.

Source 

ServiceExtensionResponse.error(int errorCode, String errorDetail)
    : _result = null,
      _errorCode = errorCode,
      _errorDetail = errorDetail {
  _validateErrorCode(_errorCode);
  if (_errorDetail is! String) {
    throw new ArgumentError.value(
        _errorDetail, "errorDetail", "Must be a String");
  }
}