anchor.diff:  abstract class FakeAsync {
     factory FakeAsync() = _FakeAsync;
   
  -  FakeAsync._();
  -
     /// Returns a fake [Clock] whose time can is elapsed by calls to [elapse] and
     /// [elapseBlocking].
     ///
  
positive.code:   int get microtaskCount;
  }
  
  class _FakeAsync extends FakeAsync {
    Duration _elapsed = Duration.ZERO;
    Duration _elapsingTo;
    Queue<Function> _microtasks = new Queue();
    Set<_FakeTimer> _timers = new Set<_FakeTimer>();
  
    _FakeAsync() : super._() {
      _elapsed;
    }
  
    @override
    Clock getClock(DateTime initialTime) =>
  
negative.code:   /// and [ZoneSpecification.scheduleMicrotask] to store callbacks for later
    /// execution within the zone via calls to [elapse].
    ///
    /// Calls [callback] with `this` as argument and returns the result returned
    /// by [callback].
    dynamic run(callback(FakeAsync self));
  
    /// Runs all remaining microtasks, including those scheduled as a result of
    /// running them, until there are no more microtasks scheduled.
  