Add dependency_override for the new implicit dependency of frontend-s…

…erver: vm_service (flutter#33495)

The new dependency was added in Dart SDK in dart-lang/sdk@077a790.
This change is needed to unblock the Dart SDK -> engine roll.

flutter_frontend_server/pubspec.yaml

@@ -131,5 +131,7 @@ dependency_overrides:
     path: ../../third_party/dart/third_party/pkg/usage
   vm:
     path: ../../third_party/dart/pkg/vm
+  vm_service:
+    path: ../../third_party/dart/pkg/vm_service
   yaml:
     path: ../../third_party/dart/third_party/pkg/yaml