Add comment explain dispatchEvent override (#60734)

packages/flutter_test/lib/src/binding.dart

@@ -467,6 +467,11 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     HitTestResult hitTestResult, {
     TestBindingEventSource source = TestBindingEventSource.device,
   }) {
+    // This override disables calling this method from base class
+    // [GestureBinding] when the runtime type is [TestWidgetsFlutterBinding],
+    // while enables sub class [LiveTestWidgetsFlutterBinding] to override
+    // this behavior and use this argument to determine the souce of the event
+    // especially when the test app is running on a device.
     assert(source == TestBindingEventSource.test);
     super.dispatchEvent(event, hitTestResult);
   }