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

pkg/http/test/response_test.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 response_test;
 
 import 'dart:async';
 
 import 'package:http/http.dart' as http;
 import 'package:unittest/unittest.dart';
@@ skipping 34 matching lines @@
       var response = new http.Response.bytes([102, 246, 248, 98, 227, 114], 200,
           headers: {'content-type': 'text/plain; charset=iso-8859-1'});
       expect(response.body, equals("föøbãr"));
     });
   });
 
   group('.fromStream()', () {
     test('sets body', () {
       var controller = new StreamController(sync: true);
       var streamResponse = new http.StreamedResponse(
-          controller.stream, 200, 13);
+          controller.stream, 200, contentLength: 13);
       var future = http.Response.fromStream(streamResponse)
-        .then((response) => response.body);
+          .then((response) => response.body);
       expect(future, completion(equals("Hello, world!")));
 
       controller.add([72, 101, 108, 108, 111, 44, 32]);
       controller.add([119, 111, 114, 108, 100, 33]);
       controller.close();
     });
 
     test('sets bodyBytes', () {
       var controller = new StreamController(sync: true);
-      var streamResponse = new http.StreamedResponse(controller.stream, 200, 5);
+      var streamResponse = new http.StreamedResponse(
+          controller.stream, 200, contentLength: 5);
       var future = http.Response.fromStream(streamResponse)
-        .then((response) => response.bodyBytes);
+          .then((response) => response.bodyBytes);
       expect(future, completion(equals([104, 101, 108, 108, 111])));
 
       controller.add([104, 101, 108, 108, 111]);
       controller.close();
     });
   });
 }