[messages] Fix sorting of diagnosticCodeValues list.

Previously, this list was sorted by the full analyzer code
(`ClassName.diagnosticName`). That made sense when the constants in
the list were referenced by `ClassName.diagnosticName`. Now that the
constants are referenced by `diag.diagnosticName`, it makes sense for
the sorting to fit.

The order of entries in this list has no user-visible effect, so from
the user's point of view there is no functional change.

Change-Id: I6a6a696468fa533d6d9d7b9f12914c070c0713af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461585
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441