Updates the repo tooling to Dart 3, now that the N-2 version has Dart 3, which allows us to use Dart 3 features (e.g., records) going forward.
To allow the update:
- Removes `break` commands from `switch`es (all done automatically with `dart fix --apply`)
- Replaces mocking of `ProcessResult` with just creating an actual `ProcessResult` since it's a `final` data class and thus can't (but also doesn't need to be) mocked.
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