Reference AppLifecycleListener from widgets library (#132995)

packages/flutter/lib/src/widgets/binding.dart

@@ -254,6 +254,11 @@ abstract mixin class WidgetsBindingObserver {
   /// documentation for the [WidgetsBindingObserver] class.
   ///
   /// This method exposes notifications from [SystemChannels.lifecycle].
+  ///
+  /// See also:
+  ///
+  ///  * [AppLifecycleListener], an alternative API for responding to
+  ///    application lifecycle changes.
   void didChangeAppLifecycleState(AppLifecycleState state) { }
 
   /// Called when a request is received from the system to exit the application.
@@ -1171,6 +1176,9 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
 /// To artificially cause the entire widget tree to be disposed, consider
 /// calling [runApp] with a widget such as [SizedBox.shrink].
 ///
+/// To listen for platform shutdown messages (and other lifecycle changes),
+/// consider the [AppLifecycleListener] API.
+///
 /// See also:
 ///
 ///  * [WidgetsBinding.attachRootWidget], which creates the root widget for the

packages/flutter/lib/src/widgets/framework.dart

@@ -1317,6 +1317,9 @@ abstract class State<T extends StatefulWidget> with Diagnosticable {
   /// To artificially cause the entire widget tree to be disposed, consider
   /// calling [runApp] with a widget such as [SizedBox.shrink].
   ///
+  /// To listen for platform shutdown messages (and other lifecycle changes),
+  /// consider the [AppLifecycleListener] API.
+  ///
   /// See also:
   ///
   ///  * [deactivate], which is called prior to [dispose].