Migrate devtools_app to null safety

[kenzieschmoll]

devtools_testing and devtools_app depend on one another, so they will need to be migrated and published at the same time.

The following packages need to be migrated before devtools_app can be migrated:

• package:ansi_up (Migrate ansi_up to null safety #3027)
• package:codicon (Migrate package:codicon to null safety. #3026)
• package:dds

[leafpetersen]

Any plans on this? cc @jacob314

[polina-c]

First tasks:

• move libraries from root to subfolders (Move libraries from root to subfolders. #3594)
• update dependencies to null safe (Update dependencies to null-safe, except ansicolor. #3614, Update ansicolor to nullsafe version #3600)
• migrate primitives

[polina-c]

Remaining tasks:

• migrate shared
• migrate service
• migrate ui
• migrate config_specific
• migrate charts
• migrate http
• Migrate analytics to null safety. #3775 (Kenzie)
• migrate screens
  • Migrate screens/app_size to null safety. #3768 (Kenzie)
  • Migrate screens/debugger to null safety. #3769 (Elliott)
  • Migrate screens/inspector to null safety. #3770 (Polina)
  • Migrate screens/logging to null safety. #3771 (Polina)
  • Migrate screens/memory to null safety. #3772 (Polina)
  • Migrate screens/network to null safety. #3773 (Ben)
  • Migrate screens/performance to null safety. #3774 (Kenzie)
  • Migrate screens/profiler to null safety. #3776 (Kenzie)
  • Migrate screens/provider to null safety. #3767 (Remi?)
  • Migrate screens/vm_developer to null safety. #3780 (Ben)
• migrate remaining in lib
• migrate devtools_test
• migrate tests
• clean up
  • remove import_of_legacy_library_into_null_safe
  • remove avoid_redundant_argument_values
  • remove web-only warnings
  • remove --no-sound-null-safety from tests

[polina-c]

The recipe to migrate separate files:

1. Run 'dart migrate --skip-import-check'
2. Deselect everything by deselecting roots (upvote the button Deselect All)
3. Ctrl+F to find the file(s) you want to migrate
4. Select the file(s) and click "Apply Migration".
5. Open the package in VSCode or Android Studio and search the files for cases where the tool may be inaccurate. Make corrections and use lint warnings to interactively clean up upstream and downstream code.

What to watch for, to find the tool inaccuracies:

1. Added types dynamic or num. Most likely you know which specific type should be here instead.
2. bool? in most cases can become bool with default value.
3. Type casts (search for as) may mean the tool did not add a generic type parameter. After adding it, the type cast will be linted as unnecessary.
4. Generic nullables (search for ?> and ?,) in most cases can be non-nullable.
5. Nullable fields may become late or late final (upvote lint).
6. Unconditional operation '!' may mean the variable should be non-nullable (upvote lint).
7. If '!' is applied to a collection, it can be coalesced with empty collection ( ?? []) instead.

[polina-c]

DevTools are sound null safe now.