Update frontend_server_client constraint to allow version 4.0.0 (#2153)

Updating to package:frontend_server_client v4.0.0 is only a breaking change for dependents that set the debug argument of FrontendServerClient.start to true, andpackage:test_core never does that.

Author: derekxu16

pkgs/test_core/CHANGELOG.md

@@ -8,6 +8,8 @@
 * **BREAKING**: Removed `Configuration.pubServeUrl` and support for it.
 * Fix running of tests defined under `lib/` with relative imports to other
   libraries in the package.
+* Update the `package:frontend_server_client` constraint to allow version
+  `4.0.0`.
 
 ## 0.5.9
 

pkgs/test_core/pubspec.yaml

@@ -13,7 +13,7 @@ dependencies:
   boolean_selector: ^2.1.0
   collection: ^1.15.0
   coverage: ^1.0.0
-  frontend_server_client: '>=3.2.0 <4.0.0'
+  frontend_server_client: '>=3.2.0 <5.0.0'
   glob: ^2.0.0
   io: ^1.0.0
   meta: ^1.3.0