Silence unnecessary cast so devtools can work with web 0.5.0 in googl…

…e3 (flutter#7179)

Note that 0.5.0 hasn't been released yet. After it rolls into google3,
we can publish and devtools can be updated to use it. This purely enables
that roll.

packages/devtools_app/lib/src/shared/config_specific/notifications/_notifications_web.dart

@@ -17,6 +17,9 @@ class Notification {
   late final web.Notification _impl;
 
   static Future<String> requestPermission() async =>
+      // TODO(srujzs): This was needed in 0.4.0 as generics were not available.
+      // This is no longer true 0.5.0 onwards.
+      // ignore: unnecessary_cast
       ((await web.Notification.requestPermission().toDart) as JSString).toDart;
 
   void close() {