`dcm check-unused-code`: bugs discovered where code is incorrectly unused.

[kenzieschmoll]

Findings:

1. vm_developer screen: this to delete the unused code requires deleting most of this file. It seems like a bug that the description is not used. CC @bkonyi

packages/devtools_app/lib/src/screens/vm_developer/object_inspector/inbound_references_tree.dart (1 issue):
    ✖ unused field description
      at packages/devtools_app/lib/src/screens/vm_developer/object_inspector/inbound_references_tree.dart:29:3

2. vm_developer screen: it seems like a bug if we are intending ProfileCode to be serializable but it is unused. CC @bkonyi

packages/devtools_app/lib/src/screens/vm_developer/vm_service_private_extensions.dart (8 issues):
    ✖ unused method toJson
      at packages/devtools_app/lib/src/screens/vm_developer/vm_service_private_extensions.dart:856:3

3. profiler screen: we used to have functionality to load an app startup profile by a user tag. This should be investigated to see if this method can be removed or if the fact that this is unused is a regression.

packages/devtools_app/lib/src/screens/profiler/cpu_profiler_controller.dart (1 issue):
    ✖ unused method loadAppStartUpProfile
      at packages/devtools_app/lib/src/screens/profiler/cpu_profiler_controller.dart:414:3