Improve efficiency of copying the animation ObserverList in notifyListeners (#102536)

Author: jason-simmons

packages/flutter/lib/src/animation/listener_helpers.dart

@@ -137,7 +137,7 @@ mixin AnimationLocalListenersMixin {
   @protected
   @pragma('vm:notify-debugger-on-exception')
   void notifyListeners() {
-    final List<VoidCallback> localListeners = List<VoidCallback>.of(_listeners);
+    final List<VoidCallback> localListeners = _listeners.toList(growable: false);
     for (final VoidCallback listener in localListeners) {
       InformationCollector? collector;
       assert(() {
@@ -226,7 +226,7 @@ mixin AnimationLocalStatusListenersMixin {
   @protected
   @pragma('vm:notify-debugger-on-exception')
   void notifyStatusListeners(AnimationStatus status) {
-    final List<AnimationStatusListener> localListeners = List<AnimationStatusListener>.of(_statusListeners);
+    final List<AnimationStatusListener> localListeners = _statusListeners.toList(growable: false);
     for (final AnimationStatusListener listener in localListeners) {
       try {
         if (_statusListeners.contains(listener))

packages/flutter/lib/src/foundation/observer_list.dart

@@ -76,6 +76,11 @@ class ObserverList<T> extends Iterable<T> {
 
   @override
   bool get isNotEmpty => _list.isNotEmpty;
+
+  @override
+  List<T> toList({bool growable = true}) {
+    return _list.toList(growable: growable);
+  }
 }
 
 /// A list optimized for the observer pattern, but for larger numbers of observers.