Are you using XMLHttpRequest directly instead of the Mendix API functions to communicate with the Mendix runtime? Also for doing Ajax requests always use either DOJO functions, JQuery functions or preferably if the actions are for executing microflows or xpath data retrievals use the Mendix api.
That should solve this error. Using the XMLHttpRequest functions isn't wrong javascript therefore JSLint won't complain. But using the function directly is considered unwanted by the MX api. The first line says it all. the rest is jibbirish that will confuse anyone :)