The docs say that Separator is a Shared property (which it should be, so it can be used independent of FileSystemItem instances), but the tooltip on hover over FileSystemItem.Separator shows it as "Readonly Property Separator As String".
I would also recommend renaming or aliasing this as PathSeparator to avoid any confusion with the same-named UI control, and also to make the name itself more specific. A rename would be a breaking change but we can handle it this early in the game I think.