Code for a timer

index.html

<span class="clock"></span>

main.dart

import 'dart:async';
import 'dart:html';

void main() {
	var hours = 0, minutes = 0, seconds = 0;
  
  updateClock(hours, minutes, seconds);
  
  Timer.periodic(Duration(seconds: 1), (_) {
    seconds++;
    
    if (seconds >= 60) {
      seconds = 0;
      minutes++;
      
      if (minutes >= 60) {
        minutes = 0;
        hours++;
      }
    }
    
  	updateClock(hours, minutes, seconds);  
  });
}

updateClock(hours, minutes, seconds) {
  var h = hours, m = minutes, s = seconds;
  
  if (hours < 10) h = '0$hours';
  if (minutes < 10) m = '0$minutes';
  if (seconds < 10) s = '0$seconds';
  
  querySelector('.clock')
    ..innerHtml = '$h:$m:$s';
}

styles.css

.clock {
  color: white;
  margin-bottom: 0;
}