Add support for frontend_server in chrome_proxy_server_test.dart

See:

I am also a bit confused why this is prefixed with `core` (unless the import is prefixed?)

_Originally posted by @jakemac53 in https://github.com/dart-lang/webdev/diffs_

Start by reading chrome_proxy_server_test.dart and identify how frontend_server is currently represented or omitted. Compare the test setup with the referenced frontend_server behavior, then add coverage for the supported case and verify the test passes.

Written by the indexing model from the issue text.