chrome uncaught type error, no method tabledrag.js::once()

marking this as major because not everyone has access to drush and not every module using tabledrag.js (including drupal core modules) degrade gracefully.

[running drupal HEAD 3ac0d1] chrome dev tools, resources, indicating line 17 misc/tabledrag.js returning:

'Uncaught TypeError: Object [object Object] has no method "once"'

not such a big deal when it affects forms that degrade gracefully, but for example, when running update.php in chrome, update does not run.

if Drupal.behaviors.tabledrag is setting a drupal variable only once, what happens in case that variable doesn't exist when called?

tabledrag.js, line 18:
// Create the new tableDrag instance. Save in the Drupal variable
// to allow other scripts access to the object.