Fix one final strong-mode warning.

lib/src/mock_client.dart

 // Copyright (c) 2012, 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 'base_client.dart';
 import 'base_request.dart';
 import 'byte_stream.dart';
 import 'request.dart';
 import 'response.dart';
 import 'streamed_response.dart';
-import 'utils.dart';
 
 // TODO(nweiz): once Dart has some sort of Rack- or WSGI-like standard for
 // server APIs, MockClient should conform to it.
 
 /// A mock HTTP client designed for use when testing code that uses
 /// [BaseClient]. This client allows you to define a handler callback for all
 /// requests that are made through it so that you can mock a server without
 /// having to send real HTTP requests.
 class MockClient extends BaseClient {
   /// The handler for receiving [StreamedRequest]s and sending
@@ skipping 40 matching lines @@
             contentLength: response.contentLength,
             request: request,
             headers: response.headers,
             isRedirect: response.isRedirect,
             persistentConnection: response.persistentConnection,
             reasonPhrase: response.reasonPhrase);
       });
     });
 
   /// Sends a request.
-  Future<StreamedResponse> send(BaseRequest request) {
+  Future<StreamedResponse> send(BaseRequest request) async {
     var bodyStream = request.finalize();
-    return async.then((_) => _handler(request, bodyStream));
+    return await _handler(request, bodyStream);
   }
 }
 
 /// A handler function that receives [StreamedRequest]s and sends
 /// [StreamedResponse]s. Note that [request] will be finalized.
 typedef Future<StreamedResponse> MockClientStreamHandler(
     BaseRequest request, ByteStream bodyStream);
 
 /// A handler function that receives [Request]s and sends [Response]s. Note that
 /// [request] will be finalized.
 typedef Future<Response> MockClientHandler(Request request);