anchor.diff:      element.querySelector('.resume').onClick.listen((_) => resume());
       element.querySelector('.cancel').onClick.listen((_) => cancel());
     }
  -  
  +
     void listenForEventA() {
       if (subscription != null) {
         appendOuput('Already listening for an event.');
  
positive.code:       appendOuput('---');
      }
    }
    
    void listenForEventB() {
      if (subscription != null) {
        appendOuput('Already listening for an event.');
  
negative.code:   Logger.root.onRecord.listen((LogRecord r) {
      print('${r.time}\t${r.loggerName}\t[${r.level.name}]:\t${r.message}');
    });
  
    // Root logger level.
    Logger.root.level = Level.FINEST;
  }
  
  class Listener {
    Element element;
  
    TextAreaElement output;
  
    StreamSubscription<String> subscription;
  
    Listener(this.element) {
      output = element.querySelector('textarea');
      // Init buttons.
  