See notes on events in Assignment. We might need to hook changes here
into a CliRequisition so they appear of the command line.
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);
    }