anchor.diff:  }
   
   class Paragraph extends LeafRenderObjectWrapper {
  -  Paragraph({ Key key, this.text }) : super(key: key);
  +  Paragraph({ Key key, this.text }) : super(key: key) {
  +    assert(text != null);
  +  }
   
     final TextSpan text;
   
  
positive.code:   BlockBody(List<Widget> children, {
      Key key,
      this.direction: BlockDirection.vertical
    }) : super(key: key, children: children);
  
    final BlockDirection direction;
  
  
negative.code: 
  class Baseline extends OneChildRenderObjectWrapper {
    Baseline({ Key key, this.baseline, this.baselineType: TextBaseline.alphabetic, Widget child })
      : super(key: key, child: child) {
      assert(baseline != null);
      assert(baselineType != null);
    }
  
    final double baseline; // in pixels
    final TextBaseline baselineType;
  