anchor.diff:  
     /// Pump the rendering pipeline to generate a frame.
     void beginFrame() {
  +    assert(renderView != null);
       RenderObject.flushLayout();
  -    _renderView.updateCompositingBits();
  +    renderView.updateCompositingBits();
       RenderObject.flushPaint();
  -    _renderView.compositeFrame();
  +    renderView.compositeFrame();
     }
   
     void hitTest(HitTestResult result, Point position) {
  -    _renderView.hitTest(result, position: position);
  +    assert(renderView != null);
  +    renderView.hitTest(result, position: position);
       super.hitTest(result, position);
     }
   }
   
   /// Prints a textual representation of the entire render tree.
   void debugDumpRenderTree() {
  -  debugPrint(Renderer.instance.renderView.toStringDeep());
  +  debugPrint(Renderer.instance?.renderView?.toStringDeep());
   }
   
   /// Prints a textual representation of the entire layer tree.
   void debugDumpLayerTree() {
  -  debugPrint(Renderer.instance.renderView.layer.toStringDeep());
  +  debugPrint(Renderer.instance?.renderView?.layer?.toStringDeep());
   }
   
   /// A concrete binding for applications that use the Rendering framework
   /// directly. This is the glue that binds the framework to the Flutter engine.
   class RenderingFlutterBinding extends BindingBase with Scheduler, Renderer, Gesturer {
     RenderingFlutterBinding({ RenderBox root }) {
  +    assert(renderView != null);
       renderView.child = root;
     }
   }
  
positive.code:       renderView = new RenderView();
        renderView.scheduleInitialFrame();
      }
      handleMetricsChanged(); // configures _renderView's metrics
    }
  
    /// The render tree that's attached to the output surface.
    RenderView get renderView => _renderView;
    RenderView _renderView;
    void set renderView(RenderView value) {
      if (_renderView == value)
        return;
      if (_renderView != null)
        _renderView.detach();
      _renderView = value;
      if (_renderView != null)
        _renderView.attach();
    }
  
    void handleMetricsChanged() {
      _renderView.rootConstraints = new ViewConstraints(size: ui.window.size);
    }
  
    void _handlePersistentFrameCallback(Duration timeStamp) {
  
negative.code:     _instance = this;
      ui.window.onMetricsChanged = handleMetricsChanged;
      initRenderView();
      assert(renderView != null);
      assert(() {
        initServiceExtensions();
        return true;
  