It turns out that my blog post of last week was completely wrong in claiming that migrating the Optimizely module from Beta 10 to Beta 13 did not require any code changes. In fact, doing so completely breaks the site as soon as the module is enabled.
I'm not sure what went wrong during the migration. I must have had a brain burp and copied the wrong release into the test directory.
In any case, I have backtracked to Beta 11 and successfully migrated to it by making one coding change.
Method checkPlain() has been moved from class