Timer constructor

Timer(Duration duration, void callback())

Creates a new timer.

The callback function is invoked after the given duration.

Implementation 

factory Timer(Duration duration, void callback()) {
  if (Zone.current == Zone.root) {
    // No need to bind the callback. We know that the root's timer will
    // be invoked in the root zone.
    return Zone.current.createTimer(duration, callback);
  }
  return Zone.current
      .createTimer(duration, Zone.current.bindCallbackGuarded(callback));
}