anchor.diff:  
   class Opacity extends OneChildRenderObjectWrapper {
     Opacity({ Key key, this.opacity, Widget child })
  -    : super(key: key, child: child);
  +    : super(key: key, child: child) {
  +    assert(opacity >= 0.0 && opacity <= 1.0);
  +  }
   
     final double opacity;
   
  
positive.code: 
  class Align extends OneChildRenderObjectWrapper {
    Align({ Key key, this.horizontal: 0.5, this.vertical: 0.5, Widget child })
      : super(key: key, child: child);
  
    final double horizontal;
    final double vertical;
  
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;
  