Description

When we package Flutter, we used to find the "current" tag (which is the version number) by starting at the revision we are building on and looking backwards in time to find the most recent tag. This causes problems on release builds when we failed to tag properly.

This PR makes the packaging script be more strict by requiring the given revision to itself have a tag, but only when we're publishing the result. When we're not publishing the result, it's more lenient, since otherwise we couldn't test packaging on non-release commits.

I also renamed ProcessRunnerException to PreparePackageException, since we were using that exception more generally than just for processes.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors (See [Test Coverage]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• No, this is not a breaking change.