In #65041, a change to dartfmt caused the customer_testing shard cocoon test to fail the formatter test. When trying to update cocoon with the new formatting, Cocoon CI failed because it is based on Flutter stable SDK.

If cocoon's test suite is included in customer_testing, then Cocoon's CI should use the latest Flutter SDK, to minimize the time Cocoon's CI is broken. If Cocoon's CI needs to use stable Flutter SDK, then it should be removed from customer_testing (which usually runs latest SDK).

The version of the Flutter SDK used in Cocoon CI is defined in https://flutter.googlesource.com/recipes/+/refs/heads/master/recipes/cocoon.py#36.