anchor.diff:  /// makes the call to `runZonedGuarded` throw that error,
   /// and otherwise the call to `runZonedGuarded` returns `null`.
   ///
  -/// The zone will always be an error-zone.
  -///
  -/// Errors will never cross error-zone boundaries by themselves.
  -/// Errors that try to cross error-zone boundaries are considered uncaught in
  -/// their originating error zone.
  -/// ```dart
  -/// var future = Future.value(499);
  -/// runZonedGuarded(() {
  -///   var future2 = future.then((_) { throw "error in first error-zone"; });
  -///   runZonedGuarded(() {
  -///     var future3 = future2.catchError((e) { print("Never reached!"); });
  -///   }, (e, s) { print("unused error handler"); });
  -/// }, (e, s) { print("catches error of first error-zone."); });
  -/// ```
  -/// Example:
  -/// ```dart
  -/// runZonedGuarded(() {
  -///   Future(() { throw "asynchronous error"; });
  -/// }, (e, s) => print(e));  // Will print "asynchronous error".
  -/// ```
  -/// It is possible to manually pass an error from one error zone to another
  -/// by re-throwing it in the new zone. If [onError] throws, that error will
  -/// occur in the original zone where [runZoned] was called.
  +/// The zone will always be an error-zone ([Zone.errorZone]), so returning
  +/// a future created inside the zone, and waiting for it outside of the zone,
  +/// will risk the future not being seen to complete.
   @Since("2.8")
   R? runZonedGuarded<R>(R body(), void onError(Object error, StackTrace stack),
       {Map<Object?, Object?>? zoneValues, ZoneSpecification? zoneSpecification}) {
  
positive.code:   /// This is the closest parent zone of this zone that provides a
    /// [handleUncaughtError] method.
    ///
    /// Asynchronous errors never cross zone boundaries between zones with
    /// different error handlers.
    ///
    /// Example:
    /// ```dart
  
negative.code: 
  part of dart.async;
  
  typedef ZoneCallback<R> = R Function();
  typedef ZoneUnaryCallback<R, T> = R Function(T);
  typedef ZoneBinaryCallback<R, T1, T2> = R Function(T1, T2);
  
  /// The type of a custom [Zone.handleUncaughtError] implementation function.
  ///
  