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: </p>
  <p>History:</p>
  <ul>
      <li *ng-for="#time of times">
          {{time}}
      </li>
  </ul>
  ''', directives: const [NgFor, NgIf])
  
negative.code:   bool isPomotime(){
      return currentState == ClockState.pomodoro;
    }
  }
  