void displayError(String errorMessage, { bool showUsage: false, CLIColor color: CLIColor.boldRed })

Source 

void displayError(String errorMessage,
    {bool showUsage: false, CLIColor color: CLIColor.boldRed}) {
  outputSink.writeln(
      "${colorSymbol(color)}$_ErrorDelimiter$errorMessage$defaultColorSymbol");
  if (showUsage) {
    outputSink.writeln("\n${options.usage}");
  }
}