future.dart

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

Completer _completer = new Completer();

void main() {
  
  MutationObserver observer = new MutationObserver(onMutation);
  observer.observe(document.body, childList: true, subtree: true);
  
  querySelector('#button').addEventListener('click', (e){
    var e = new DivElement();
    e.text = "Some element";
    document.body.append(e);
  });
  
  _completer.future.timeout(const Duration(seconds: 5), onTimeout: () => false).then((value) {
    print("value is ${value}");
  });
}

void onMutation(List mutations, MutationObserver observer) {
  for(var mutation in mutations){
    print(mutation.addedNodes[0]);
  }
  _completer.complete(true);
}