The flutter/packages repository needs to test with both Flutter master and Flutter stable, and sometimes specific tests need to know which mode they are running under (e.g,. tests with golden files or that rely on auto-formatting need to be skipped on one of the two channels, because it's periodically impossible to pass on both).

With Cirrus, we put the channel into the environment, so scripts just check Platform.environment['CHANNEL']. Ideally we would have the LUCI recipe propagate the existing channel property from the CI config into the environment as CHANNEL so everything would keep working the same way, but I'm open to other options.