fixes #2533165 slider unit test fails in IE9

I tested the unit test
yui3/src/slider/tests/unit/assets/slider-tests.js
and all tests pass in the following VM browsers/OS:
IE6 - WinXP
IE7 - WinXP
IE8 - WinXP
IE9 - WinXP
IE10 - Win8

and current versions of these browsers natively on mac
Firefox
Chrome
Safari