The API doc says:

sender: The player or server on whose behalf the commands should be executed.

Examples use server but with Spigot I can't get server to work anywhere, only player. I'm getting error:

Cannot cast org.bukkit.craftbukkit.v1_10_R1.CraftServer to org.bukkit.command.CommandSender.

There's a forum post that says this work(s)(ed) with Canary so I'm guessing the Bukkit code was never enhanced to support a CraftServer object.

I haven't checked the source yet but will soon. Just reporting this here for now.

With Bukit, if the object is a Server, we need to use server.getConsoleSender().

Here is a small fix that works; in the slash.js file (on the module folder) change the slash function to

function slash( commands, sender ){
  if (_.isArray(commands)){
    _.each(commands, function(command){
      slash(command, sender);
    });
    return;
  }
  if (__plugin.canary){
    if (sender === server){
      server.consoleCommand( commands );
    } else {
      server.consoleCommand( commands, sender );
    }
  }
  if (__plugin.bukkit){
      if (sender === server){
      server.dispatchCommand(server.getConsoleSender(), commands );
    } else {
      server.dispatchCommand(sender, commands);
    }
    
  }
}