Add utilities for different base directories

[4e554c4c]

See related dart-lang/sdk#41560, https://github.com/dart-lang/sdk/issues/49166#issuecomment-1223943236

Test plan:

$ dart test
00:01 +16: All tests passed!

run dart doc and inspect docs for correctness.

---

• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[4e554c4c]

note: the comment by the google bot is out of date. I have signed the CLA

[sortie]

The contributor pinged me out of band asking for a reviewer, can you route this pull request appropriately @kevmoo?

[kevmoo]

@devoncarew ?

[4e554c4c]

ping :)

[kevmoo]

you'll have to rebase this. @devoncarew – thoughts?

[4e554c4c]

you'll have to rebase this.

done

[emersion]

Gentle ping @devoncarew, this would be very nice to have :)