Constructor property map always contains conditionally assigned properties

Loading and running jquery 1.8.3 on top of env.js throws the following exception for an undefined function:

    jquery-1.8.3.js:2952 TypeError: Cannot call undefined

This is because of a conditionally assigned property in the jQuery.Event constructor (line 3241). Even though the constructed object does not always contain the "isDefaultPrevented" property the property map of the created object always contains it.