Update package:frontend_server_client constraint to ^4.0.0

Author: derekxu16

build_runner/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 2.4.9-wip
 
+- Update the `package:frontend_server_client` constraint to `^4.0.0`.
+
 ## 2.4.8
 
 - Update README.md to point to the FAQ and other docs.

build_runner/pubspec.yaml

@@ -24,7 +24,7 @@ dependencies:
   collection: ^1.15.0
   crypto: ^3.0.0
   dart_style: ^2.0.0
-  frontend_server_client: ^3.0.0
+  frontend_server_client: ^4.0.0
   glob: ^2.0.0
   graphs: ^2.2.0
   http_multi_server: ^3.0.0