anchor.diff:      this.scrollPhysics,
       this.scrollDirection = Axis.horizontal,
       this.customSize,
  +    this.allowImplicitScrolling = false,
     })  : pageOptions = null,
           assert(itemCount != null),
           assert(builder != null),
  
positive.code: 
    /// The axis along which the [PageView] scrolls. Mirror to [PageView.scrollDirection]
    final Axis scrollDirection;
  
    bool get _isBuilder => builder != null;
  
  
negative.code:     this.scrollPhysics,
      this.scrollDirection = Axis.horizontal,
      this.customSize,
      this.allowImplicitScrolling = false,
    })  : itemCount = null,
          builder = null,
          super(key: key);
  