anchor.diff:    Zone _zone;
   
     int _rafId = 0;
  -  Frame _currentFrame;
  -  Frame _nextFrame;
  +  Frame _currentFrame = new Frame();
  +  Frame _nextFrame = new Frame();
   
     DOMScheduler() {
       _zoneSpec = new ZoneSpecification(scheduleMicrotask: _scheduleMicrotask);
  
positive.code: /// tasks.
  library dom_scheduler;
  
  // TODO: swap and reset frame objects.
  
  import 'dart:async';
  import 'dart:collection';
  import 'dart:html' as html;
  
negative.code: 
    /// [Frame] that contains tasks for the current animation frame
    Frame get currentFrame {
      assert(_running);
      return _currentFrame;
    }
  
    /// [Frame] that contains tasks for the next animation frame.
    Frame get nextFrame {
      _requestAnimationFrame();
      return _nextFrame;
    }
  
  