anchor.diff:  // Use of this source code is governed by a BSD-style license that can be
   // found in the LICENSE file.
   
  -// @dart=2.9
  +// ignore_for_file: import_of_legacy_library_into_null_safe
   
   import 'package:flutter/material.dart';
   
  
positive.code: mixin CollapsibleAnimationMixin<T extends StatefulWidget>
      on TickerProviderStateMixin<T> {
    /// Animation controller for animating the expand/collapse icon.
    AnimationController expandController;
  
    /// An animation that rotates the expand arrow
    /// from pointing right (0.75 full turns) to pointing down (1.0 full turns).
    Animation<double> expandArrowAnimation;
  
    /// A curved animation that matches [expandController], moving from 0.0 to 1.0
    /// Useful for animating the size of a child that is appearing.
    Animation<double> expandCurve;
  
    /// Visibility state of the collapsible.
    ///
  
negative.code: 
    @override
    void didUpdateWidget(Widget oldWidget) {
      super.didUpdateWidget(oldWidget as T);
      if (isExpanded) {
        expandController.forward();
      } else {
  