`dcm check-unused-code` false positives

Command: `dcm check-unused-code packages/devtools_app/ --exclude-public-api`

False positives (one common theme I see here is that anything used in `assert` statements is falsely determined "unused"):
```
packages/devtools_app/test/test_infra/utils/extent_delegate_utils.dart (1 issue):
✖ unused field _renderObjectInitialized
at packages/devtools_app/test/test_infra/utils/extent_delegate_utils.dart:19:3

packages/devtools_app/lib/src/extensions/embedded/_controller_web.dart (1 issue):
✖ unused field _initialized
at packages/devtools_app/lib/src/extensions/embedded/_controller_web.dart:81:3

packages/devtools_app/lib/src/framework/home_screen.dart (2 issues):
✖ unused method _debugInitVmServiceCache
at packages/devtools_app/lib/src/framework/home_screen.dart:174:3

packages/devtools_app/lib/src/app.dart (2 issues):
✖ unused class _AlternateCheckedModeBanner
at packages/devtools_app/lib/src/app.dart:616:1
✖ unused constructor _AlternateCheckedModeBanner
at packages/devtools_app/lib/src/app.dart:622:3

packages/devtools_app/lib/src/screens/performance/panes/timeline_events/perfetto/_perfetto_controller_web.dart (1 issue):
✖ unused field _initialized
at packages/devtools_app/lib/src/screens/performance/panes/timeline_events/perfetto/_perfetto_controller_web.dart:162:3
```

I will add more false positives to this list as I find them (working through different directories https://github.com/flutter/devtools/issues/9906).

@incendial

Start by running `dcm check-unused-code packages/devtools_app/ --exclude-public-api` and inspect the reported Dart files, including `extent_delegate_utils.dart`, `_controller_web.dart`, `home_screen.dart`, `app.dart`, and `_perfetto_controller_web.dart`. Trace the listed fields, methods, and classes through their assert usages, then verify that genuine uses are no longer reported while actual unused code still is.

索引モデルが issue の本文から書いたものです。