dart2js: NNBD migration cleanup tasks

[rakudrama]

This is a placeholder issue for cleanup tasks from the migration of dart2js (pkg/compiler) to null safety.
Small cleanup tasks are described in TODO(48820) comments that reference this issue.

[sigmundch]

Additional tracking besides the TODOs:

• cleanup remvoving the --no-sound-null-safety flags introduced in https://dart-review.googlesource.com/c/sdk/+/261140