anchor.diff:  
   class SizeObserver extends OneChildRenderObjectWrapper {
     SizeObserver({ Key key, this.callback, Widget child })
  -    : super(key: key, child: child);
  +    : super(key: key, child: child) {
  +    assert(callback != null);
  +  }
   
     final SizeChangedCallback callback;
   
  
positive.code: 
  class Transform extends OneChildRenderObjectWrapper {
    Transform({ Key key, this.transform, this.origin, Widget child })
      : super(key: key, child: child);
  
    final Matrix4 transform;
    final Offset origin;
  
negative.code: 
  class Align extends OneChildRenderObjectWrapper {
    Align({ Key key, this.horizontal: 0.5, this.vertical: 0.5, Widget child })
      : super(key: key, child: child) {
      assert(horizontal != null);
      assert(vertical != null);
    }
  
    final double horizontal;
    final double vertical;
  