link.prepend is not a function in older browsers

Seen in logstash for Chrome 50.
Icons are not essentially, so gracefully degrade in these browsers
to items without icons.

Change-Id: I083c87c9bf5f256c8bcd2893e251571b97440ce1

resources/skins.vector.js/dropdownMenus.js

@@ -102,7 +102,8 @@ function addPortletLinkHandler( item, data ) {
 		}
 	}
 
-	if ( link && iconElement ) {
+	// Check link.prepend exists for older browser since this is ES5 code
+	if ( link && iconElement && link.prepend ) {
 		link.prepend( iconElement );
 	}