[flutter_markdown] update markdown dependency

[domesticmouse]

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan]

Looks like the update breaks one of our test expectations.

[domesticmouse]

Any ideas why Cirrus is showing tests as failing when the tests complete successfully on a local build?

[stuartmorgan]

Have you tried deleting your lockfile to see if you have an older version of a transitive dependency that could be causing the difference? That's the first thing that comes to mind as a potential difference, since CI runs are always clean, so should always have the latest resolvable dependencies.

[domesticmouse]

Failure caused by change of behaviour in markdown package. Reported in dart-lang/tools#1300

[domesticmouse]

The tests should now turn green.

[stuartmorgan]

LGTM