anchor.diff:    /// Once a [Timer] has been canceled, the callback function will not be called
     /// by the timer. Calling [cancel] more than once on a [Timer] is allowed, and
     /// will have no further effect.
  +  ///
  +  /// Example:
  +  /// ```dart
  +  /// final timer =
  +  ///     Timer(const Duration(seconds: 5), () => print('Timer finished'));
  +  /// // Cancel timer, callback never called.
  +  /// timer.cancel();
  +  /// ```
     void cancel();
   
     /// The number of durations preceding the most recent timer event.
  
positive.code: 
  part of dart.async;
  
  /// A count-down timer that can be configured to fire once or repeatedly.
  ///
  /// The timer counts down from the specified duration to 0.
  /// When the timer reaches 0, the timer invokes the specified callback function.
  
negative.code:   /// scheduled for - even if the actual callback was delayed.
    ///
    /// [duration] must a non-negative [Duration].
    ///
    /// Example:
    /// ```dart
    /// var counter = 3;
    /// Timer.periodic(const Duration(seconds: 2), (timer) {
    ///   print(timer.tick);
    ///   counter--;
    ///   if (counter == 0) {
    ///     print('Cancel timer');
    ///     timer.cancel();
    ///   }
    /// });
    /// // Outputs:
    /// // 1
    /// // 2
    /// // 3
    /// // "Cancel timer"
    /// ```
    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
  