Use ServicesBinding.rootIsolateToken to check for main isolate in f…

…lutter

flutter/lib/src/sentry_flutter.dart

@@ -121,6 +121,11 @@ mixin SentryFlutter {
     options.addEventProcessor(PlatformExceptionEventProcessor());
 
     _setSdk(options);
+
+    // ignore: invalid_use_of_internal_member
+    options.isMainIsolate = () {
+      return ServicesBinding.rootIsolateToken != null;
+    };
   }
 
   /// Install default integrations