Make BaseRequest.contentType use null rather than -1 as a flag value.

pkg/http/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.
 
 library mock_client;
 
 import 'dart:async';
 
 import 'base_client.dart';
 import 'base_request.dart';
@@ skipping 28 matching lines @@
             ..maxRedirects = baseRequest.maxRedirects
             ..headers.addAll(baseRequest.headers)
             ..bodyBytes = bodyBytes
             ..finalize();
 
         return fn(request);
       }).then((response) {
         return new StreamedResponse(
             new ByteStream.fromBytes(response.bodyBytes),
             response.statusCode,
-            response.contentLength,
+            contentLength: response.contentLength,
             request: baseRequest,
             headers: response.headers,
             isRedirect: response.isRedirect,
             persistentConnection: response.persistentConnection,
             reasonPhrase: response.reasonPhrase);
       });
     });
 
   /// Creates a [MockClient] with a handler that receives [StreamedRequest]s and
   /// sends [StreamedResponse]s.
   MockClient.streaming(MockClientStreamHandler fn)
     : this._((request, bodyStream) {
       return fn(request, bodyStream).then((response) {
         return new StreamedResponse(
             response.stream,
             response.statusCode,
-            response.contentLength,
+            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) {
     var bodyStream = request.finalize();
     return async.then((_) => _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);