deprecate app.experimental

lib/application.js

@@ -119,6 +119,9 @@ app.use = function(fn){
  */
 
 app.callback = function(){
+  if (this.experimental) {
+    console.error('Experimental ES7 Async Function support is deprecated. Please look into Koa v2 as the middleware signature has changed.')
+  }
   var fn = this.experimental
     ? compose_es7(this.middleware)
     : co.wrap(compose(this.middleware));