anchor.diff:      if (computation != null) {
         result = result.then((ignored) => computation());
       }
  -    new Timer(duration, () { completer.complete(null); });
  +    new Timer(duration, completer.complete);
       return result;
     }
   
  
positive.code:    * If calling [computation] throws, the created future will complete with the
     * error.
     *
     * See also [Completer] for a way to complete a future at a later
     * time that isn't a known fixed duration.
     */
    factory Future.delayed(Duration duration, [T computation()]) {
      Completer completer = new Completer.sync();
  
negative.code:   }
  
    /**
     * Creates a future that runs its computation after a delay.
     *
     * The [computation] will be executed after the given [duration] has passed,
     * and the future is completed with the result.
     * If the duration is 0 or less,
     * it completes no sooner than in the next event-loop iteration.
     *
     * If [computation] is omitted,
     * it will be treated as if [computation] was set to `() => null`,
  