anchor.diff:  class IntTween extends Tween<int> {
     /// Creates an int tween.
     ///
  -  /// The [begin] and [end] arguments must not be null.
  +  /// The [begin] and [end] properties must be non-null before the tween is
  +  /// first used, but the arguments can be null if the values are going to be
  +  /// filled in later.
     IntTween({ int begin, int end }) : super(begin: begin, end: end);
   
     // The inherited lerp() function doesn't work with ints because it multiplies
  
positive.code: class RectTween extends Tween<Rect> {
    /// Creates a rect tween.
    ///
    /// The [begin] and [end] arguments must not be null.
    RectTween({ Rect begin, Rect end }) : super(begin: begin, end: end);
  
    @override
  
negative.code: class SizeTween extends Tween<Size> {
    /// Creates a size tween.
    ///
    /// The [begin] and [end] properties must be non-null before the tween is
    /// first used, but the arguments can be null if the values are going to be
    /// filled in later.
    SizeTween({ Size begin, Size end }) : super(begin: begin, end: end);
  
    @override
  