`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.

Written by the indexing model from the issue text.