anchor.diff:  /// * [ClampingScrollPhysics], which provides the clamping overscroll behavior
   ///   found on Android.
   class AlwaysScrollableScrollPhysics extends ScrollPhysics {
  +  /// Creates scroll physics that always lets the user scroll.
     const AlwaysScrollableScrollPhysics({ ScrollPhysics parent }) : super(parent);
   
     @override
  
positive.code: /// * [ClampingScrollPhysics], which is the analogous physics for Android's
  ///   clamping behavior.
  class BouncingScrollPhysics extends ScrollPhysics {
    const BouncingScrollPhysics({ ScrollPhysics parent }) : super(parent);
  
    @override
  
negative.code: ///   provide the glowing effect that is usually found with this clamping effect
  ///   on Android.
  class ClampingScrollPhysics extends ScrollPhysics {
    /// Creates scroll physics that prevent the scroll offset from exceeding the
    /// bounds of the content..
    const ClampingScrollPhysics({ ScrollPhysics parent }) : super(parent);
  
    @override
  