anchor.diff:    void addToHistory(){
       switch(currentState){
         case ClockState.pomodoro:
  -        times.add("rest");
  +        var now = new DateTime.now();
  +        var start = now.subtract(new Duration(seconds: startRestAt));
  +        var end = now;
  +        periods.add(new Period(start, end,"rest"));
           break;
         case ClockState.rest:
  -        times.add("pomodoro");
  +        var now = new DateTime.now();
  +        var start = now.subtract(new Duration(seconds: startPomodoroAt));
  +        var end = now;
  +        periods.add(new Period(start, end,"pomodoro"));
           break;
       }
     }
  
positive.code:     counter= prettyPrintTime(25*60);
      currentState = ClockState.pomodoro;
      currentDuration = startPomodoroAt;
      times = new List();
    }
    
    void start() {
  
negative.code:   String counter;
    Stopwatch watch = new Stopwatch();
    Timer timer;
    List<Period> periods;
  
    ClockState currentState;
  
  