Timer.periodic(Duration duration, void callback(Timer timer))

Creates a new repeating timer.

The callback is invoked repeatedly with duration intervals until canceled with the cancel function.

The exact timing depends on the underlying timer implementation. No more than n callbacks will be made in duration * n time, but the time between two consecutive callbacks can be shorter and longer than duration.

In particular, an implementation may schedule the next callback, e.g., a duration after either when the previous callback ended, when the previous callback started, or when the previous callback was scheduled for - even if the actual callback was delayed.

Source 

factory Timer.periodic(Duration duration,
                       void callback(Timer timer)) {
  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.createPeriodicTimer(duration, callback);
  }
  // TODO(floitsch): the return type should be 'void', and the type
  // should be inferred.
  var boundCallback = Zone.current.bindUnaryCallback/*<dynamic, Timer>*/(
      callback, runGuarded: true);
  return Zone.current.createPeriodicTimer(duration, boundCallback);
}