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