anchor.diff:  class Split extends StatefulWidget {
     /// Builds a split oriented along [axis].
     Split({
  -    Key key,
  -    @required this.axis,
  -    @required this.children,
  -    @required this.initialFractions,
  +    Key? key,
  +    required this.axis,
  +    required this.children,
  +    required this.initialFractions,
       this.minSizes,
       this.splitters,
  -  })  : assert(axis != null),
  -        assert(children != null && children.length >= 2),
  -        assert(initialFractions != null && initialFractions.length >= 2),
  +  })  : assert(children.length >= 2),
  +        assert(initialFractions.length >= 2),
           assert(children.length == initialFractions.length),
           super(key: key) {
       _verifyFractionsSumTo1(initialFractions);
       if (minSizes != null) {
  -      assert(minSizes.length == children.length);
  +      assert(minSizes!.length == children.length);
       }
       if (splitters != null) {
  -      assert(splitters.length == children.length - 1);
  +      assert(splitters!.length == children.length - 1);
       }
     }
   
  
positive.code: }
  
  class _SplitState extends State<Split> {
    List<double> fractions;
  
    bool get isHorizontal => widget.axis == Axis.horizontal;
  
  
negative.code:               // drag action triggers which is't ideal but isn't a launch blocker.
                dragStartBehavior: DragStartBehavior.down,
                child: widget.splitters != null
                    ? widget.splitters![i]
                    : DefaultSplitter(isHorizontal: isHorizontal),
              ),
            ),
  