[analyzer] `unused_import` not being triggered

[HosseinYousefi]

Specifically in this file, unused dart:lib imports like dart:ffi or dart:async won't trigger the unused_import lint. Unused package:pkg/pkg.dart and path/to/file.dart imports do trigger the lint correctly!

- Dart 3.4.0-242.0.dev (dev) (Sat Mar 16 17:06:53 2024 -0700) on "macos_arm64"
- on macos / Version 14.4.1 (Build 23E224)
- locale is en