function async(fn) {
      fn && $$asyncCallback(fn);
    }
@ngdoc method
@name $animateProvider#classNameFilter

@description
Sets and/or returns the CSS class regular expression that is checked when performing
an animation. Upon bootstrap the classNameFilter value is not set at all and will
therefore enable $animate to attempt to perform an animation on any element.
When setting the classNameFilter value, animations will only be performed on elements
that successfully match the filter expression. This in turn can boost performance
for low-powered devices as well as applications containing a lot of structural operations.
@param {RegExp=} expression The className expression which will be checked against all animations
@return {RegExp} The current CSS className expression value. If null then there is no expression value