logger(logger) {
      if (arguments.length) {
        this._log = logger;
        return this;
      } else {
        return this._log;
      }
    }
Get or set the logger instance used to log messages. If no arguments are
provided, returns the current logger instance. Otherwise, sets the logger
and return this Dataflow instance. Provided loggers must support the full
API of logger objects generated by the vega-util logger method. Note that
by default the log level of the new logger will be used; use the logLevel
method to adjust the log level as needed.