Description

In order to allow us to check in CI that nothing untoward happened during a run, it would be nice to have a --fatal-warnings flag to the flutter tool that would change the exit code to a non-zero exit if there had been any warnings printed to the logger during that run.

This would allow things like outdated runner deprecations warnings to break the build so that we could be sure that things were up to date, and not causing warnings that are unheeded in the repo.