This CLI log is produced by wildfly-core-2.0.0.Alpha11
[olubyans@fedovo bin]$ ./jboss-cli.sh
You are disconnected at the moment. Type 'connect' to connect to the server or 'help' for the list of supported commands.
[disconnected /] echo

[disconnected /] set a=aa
[disconnected /] echo $a
aa

And this one is the current one (with the new Aesh)
[disconnected /] set a=aa
You are disconnected at the moment. Type 'connect' to connect to the server or 'help' for the list of supported commands.
[disconnected /] echo $a

You are disconnected at the moment. Type 'connect' to connect to the server or 'help' for the list of supported commands.
[disconnected /]

I've verified that the variables are set with set command. I guess Aesh is handling $xxx itself and just replaces them with nothing, as Aesh is unaware of the CLI variables.
There is EchoTestCase but, as it's not interactive, Aesh does not get involved there, I guess.