pkg/shelf: use async and enable compression in example

example/example_server.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as io;
 
-void main() {
+main() async {
   var handler = const shelf.Pipeline()
       .addMiddleware(shelf.logRequests())
       .addHandler(_echoRequest);
 
-  io.serve(handler, 'localhost', 8080).then((server) {
-    print('Serving at http://${server.address.host}:${server.port}');
-  });
+  var server = await io.serve(handler, 'localhost', 8080);
+
+  // Enable content compression
+  server.autoCompress = true;
+
+  print('Serving at http://${server.address.host}:${server.port}');
 }
 
 shelf.Response _echoRequest(shelf.Request request) {
   return new shelf.Response.ok('Request for "${request.url}"');
 }