I have noticed shape://arrow in the highway.mss file (line 244) in the Open Streets DC TileMill examples.
Where in the directory structure is this url pointing to? How does this url work to get the arrow symbol?
Looks like this is a Mapnik reference.
CartoCSS is actually converted into an XML format that Mapnik uses to render the stylings of the map.
And it looks like the built-in arrow marker type "may" (their word not mine) now be referenced by shape://arrow. This being the new way to symbolize the marker as either an arrow (shape://arrow) or an ellipse (shape://ellipse), replacing MarkersSymbolizer marker-type.