Fix errors in dart:io tests found by the analyzer

tests/standalone/io/http_client_request_test.dart

 // Copyright (c) 2013, 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:io";
 import "dart:typed_data";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 void testClientRequest(Future handler(request)) {
   HttpServer.bind("127.0.0.1", 0).then((server) {
     server.listen((request) {
       request.listen((_) {}, onDone: () {
@@ skipping 89 matching lines @@
     }, test: (e) => e is HttpException);
     return request.done;
   });
 }
 
 void main() {
   testResponseDone();
   testBadResponseAdd();
   testBadResponseClose();
 }