code: function(text) {
          if (text == null) {
              throw new Error('Illegal text for Argument: ' + text);
          }
          var ev = { argument: this, oldText: this.text, text: text };
          this.text = text;
          this.emitter._dispatchEvent('argumentChange', ev);
      }
docs: See notes on events in Assignment. We might need to hook changes here
  into a CliRequisition so they appear of the command line.