Effects: fix backwards compatibility in animateClass for jQuery Core <1.8

Author: mikesherov

ui/jquery.ui.effect.js

@@ -754,6 +754,15 @@ function styleDifference( oldStyle, newStyle ) {
 	return diff;
 }
 
+// support: jQuery <1.8
+if ( !$.fn.addBack ) {
+	$.fn.addBack = function( selector ) {
+		return this.add( selector == null ?
+			this.prevObject : this.prevObject.filter( selector )
+		);
+	};
+}
+
 $.effects.animateClass = function( value, duration, easing, callback ) {
 	var o = $.speed( duration, easing, callback );