Add utilities for different base directories

[4e554c4c]

See related dart-lang/sdk#41560, dart-lang/sdk#49166 (comment)

Test plan:

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

run dart doc and inspect docs for correctness.

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

[google-cla]

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[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note: the comment by the google bot is out of date.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 :)

[mosuem]

Closing as the dart-lang/cli_util repository is merged into the dart-lang/tools monorepo. Please re-open this PR there!