After a few years now of ArcGIS Online, there is still no support for non-simple line symbols. This means for utilities, or streams, routes or whatever line features you may be managing, you cannot easily display them with flow direction arrows as in Desktop. Please Note: Those with access to ArcGIS for Server can certainly publish map services that will support this type of symbology and can be displayed through ArcGIS Online. However there are probably many that do not have that option, so in this post I'll discuss a one method as a work-around to solve this...