pkg/shelf: use async and enable compression in example

test/message_test.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 'dart:async';
 import 'dart:convert';
 
 import 'package:shelf/src/message.dart';
-import 'package:shelf/src/response.dart';
 import 'package:test/test.dart';
 
 import 'test_util.dart';
 
 class _TestMessage extends Message {
   _TestMessage(Map<String, String> headers, Map<String, Object> context, body,
       Encoding encoding)
       : super(body, headers: headers, context: context, encoding: encoding);
 
   Message change({Map<String, String> headers, Map<String, Object> context,
@@ skipping 226 matching lines @@
     test("overwrites an existing charset if given an explicit encoding", () {
       var request = _createMessage(
           body: "è",
           encoding: LATIN1,
           headers: {'content-type': 'text/plain; charset=whatever'});
       expect(request.headers,
           containsPair('content-type', 'text/plain; charset=iso-8859-1'));
     });
   });
 }