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

pkg/http/lib/src/streamed_response.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 streamed_response;
 
 import 'dart:async';
 
 import 'byte_stream.dart';
 import 'base_response.dart';
 import 'base_request.dart';
 import 'utils.dart';
 
 /// An HTTP response where the response body is received asynchronously after
 /// the headers have been received.
 class StreamedResponse extends BaseResponse {
   /// The stream from which the response body data can be read. This should
   /// always be a single-subscription stream.
   final ByteStream stream;
 
   /// Creates a new streaming response. [stream] should be a single-subscription
   /// stream.
   StreamedResponse(
       Stream<List<int>> stream,
       int statusCode,
-      int contentLength,
-      {BaseRequest request,
+      {int contentLength,
+       BaseRequest request,
        Map<String, String> headers: const {},
        bool isRedirect: false,
        bool persistentConnection: true,
        String reasonPhrase})
     : super(
         statusCode,
-        contentLength,
+        contentLength: contentLength,
         request: request,
         headers: headers,
         isRedirect: isRedirect,
         persistentConnection: persistentConnection,
         reasonPhrase: reasonPhrase),
       this.stream = toByteStream(stream);
 }