Remove dynamic calls (#125238)

I don't know what I was thinking when I wrote this code back in 2017 but this is so trivially fixable...

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

@@ -261,15 +261,11 @@ class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator>
           }
         }
       }
-    } else if (notification is ScrollEndNotification || notification is ScrollUpdateNotification) {
-      // Using dynamic here to avoid layer violations of importing
-      // drag_details.dart from gestures.
-      // ignore: avoid_dynamic_calls
-      if ((notification as dynamic).dragDetails != null) {
+    } else if ((notification is ScrollEndNotification && notification.dragDetails != null) ||
+               (notification is ScrollUpdateNotification && notification.dragDetails != null)) {
       _leadingController!.scrollEnd();
       _trailingController!.scrollEnd();
     }
-    }
     _lastNotificationType = notification.runtimeType;
     return false;
   }