anchor.diff:  
   part of dart.async;
   
  -/// A count-down timer that can be configured to fire once or repeatedly.
  +/// A countdown 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.
  
positive.code:   /// scheduled for - even if the actual callback was delayed.
    ///
    /// [duration] must a non-negative [Duration].
    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
  
negative.code:   /// 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.
  