option to validate that pubspec.lock is up to date

[DetachHead]

according to the docs, dart pub get uses the pubspec.lock file but will update it if it's outdated. however i want to be able to add a check in my CI that ensures that pubspec.lock is up to date, so this behavior is not ideal.

other package managers have this feature. some examples are npm ci and poetry lock --check

[sigurdm]

#3482 (still in draft) is introducing dart pub get --enforce-lockfile with a behaviour close to npm ci (it will fetch dependencies, but never upgrade).

Duplicate of #2890