There are two types of responders:
flash.net.Responder which is in Flash Player
mx.rpc.IResponder which is a Flex class
So, if you want to be able to use mx.rpc.IResponder (a Flex class) you need to have a Flex build of FlexUnit. This functionality is not included in the ActionScript only build as it is not part of the ActionScript API.
If you have a Flex build the Async class will have:
Async.asyncResponder() which returns an IResponder
Async.asyncNativeResponder() will return a flash.net.Responder, hence your error.