diff --git a/testing/scenario_app/lib/src/platform_echo_mixin.dart b/testing/scenario_app/lib/src/platform_echo_mixin.dart
index 7d9e5d771dbdc..2567fa1cd2744 100644
--- a/testing/scenario_app/lib/src/platform_echo_mixin.dart
+++ b/testing/scenario_app/lib/src/platform_echo_mixin.dart
@@ -9,6 +9,7 @@ import 'scenario.dart';
 
 /// Echo platform messages back to the sender.
 mixin PlatformEchoMixin on Scenario {
+  /// Handle a platform message.
   @override
   void onPlatformMessage(
     String name,