Ugh. So it turns out you can't just do a shadow-pass on an otherwise invisible plane in Blender anymore using Cycles. This is apparently because Cycles only understands "light", not shadows. If you enbiggen the image above you'll see that our little robot does cast a shadow. But that square thing is sits on does not belong there. So I'm gonna have to render out all this nonsense and then so a crazytime job of creating an alpha channel and blah-blah-blah. This is really annoying because that's a major bit of functionality which is gone from Blender.
The environmental lighting seems much better in Cycles. But I really wish there were an easier workaround with this dang shadow issue.