Gist is a part of blog entry at http://spiechu.pl/2013/01/07/pierwsze-kroki-z-dart/ [in Polish] Working version can be found at http://spiechu.pl/dart/num_counter.html

num_counter.dart

import 'dart:isolate';
import 'dart:html';
 
/**
 * Isolate running asynchronously.
 */
void numCounter() {
  port.receive((msg, replyTo) {
    Element h1 = query("h1#counter${msg['id']}");
    int counter = 0;
    int currentNumber = 0;
    new Timer.repeating(25, (Timer timer) {
      if (currentNumber > 9) {
        currentNumber = 0;
      }
      h1.text = currentNumber.toString();
      currentNumber++;
      counter++;
      if (counter > msg['counter']) {
        timer.cancel();
        h1.text = msg['number'];
        replyTo.send(msg['number']);
      }
    });
  });
}
 
/**
 * Script entry point.
 */
void main() { 
  Element submitInput = query('input#submit');
  Element errorBox = query('span#errorbox');
  Element countersDiv = query('div#counters');
  submitInput.on.click.add((e) {
    errorBox.text = '';
    countersDiv.text = '';
    HTMLInputElement inputNumbers = query('input#numbers');
    String inputtedNumbers = inputNumbers.value;
    List<String> numbers = inputtedNumbers.splitChars();
    bool hasErrors = false;
    numbers.forEach((element) {
      try {
        int.parse(element);
      } on FormatException {
         hasErrors = true;
         print('element $element is not an integer');
      }
    });
    
    if (hasErrors) {
      errorBox.text = 'Wpisz same cyfry!';
    }
    else {
      for (num i = 1; i <= numbers.length; i++) {
        HeadingElement h1 = new HeadingElement.h1();
        h1.id = 'counter$i';
        h1.text = '0';
        countersDiv.append(h1);
      }
      
      List<SendPort> spawned = new List<SendPort>();
  
      num timeout = 20;
      num firstId = 1;
  
      numbers.forEach((item) {
        SendPort sender = spawnFunction(numCounter);
        sender.call({
          'number' : item,
          'counter' : timeout,
          'id' : firstId
        }).then((String res) => print('result was: [$res]'));
        spawned.add(sender);
        timeout += 10;
        firstId += 1;
      });
    }
  });
}