Inside `CalendarBase` in the `_isNumInList` function, there is a for...in loop that does not do a `hasOwnProperty` check for each property on the Array it is iterating through, which will obviously
break with a TypeError if the environment has modified the native Array prototype with a key that has not been set to `enumerable: false`. It should either be changed to a traditional for loop or it
should do a `hasOwnProperty` check.

Regardless of your opinion on the issue of extending host/native prototypes in your runtime, YUI is a framework, and a framework should not assume that the host environment has not been modified.
Especially with something (unfortunately) as common as native prototype modifications.