anchor.diff:    int get microtaskCount;
   }
   
  -class _FakeAsync extends FakeAsync {
  +class _FakeAsync implements 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) =>
  
positive.code: abstract class FakeAsync {
    factory FakeAsync() = _FakeAsync;
  
    FakeAsync._();
  
    /// Returns a fake [Clock] whose time can is elapsed by calls to [elapse] and
    /// [elapseBlocking].
    ///
  
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.
  