code: function logWarning(terminal: Terminal, format: string, ...args: Array<mixed>): void {
    const str = util.format(format, ...args);
    terminal.log('%s: %s', chalk.yellow('warning'), str);
  }
docs: A standard way to log a warning to the terminal. This should not be called
  from some arbitrary packager logic, only from the reporters. Instead of
  calling this, add a new type of ReportableEvent instead, and implement a
  proper handler in the reporter(s).