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

pkg/http/lib/src/io_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 io_client;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'base_client.dart';
 import 'base_request.dart';
 import 'streamed_response.dart';
 
 /// A `dart:io`-based HTTP client. This is the default client.
 class IOClient extends BaseClient {
   /// The underlying `dart:io` HTTP client.
   HttpClient _inner;
 
   /// Creates a new HTTP client.
   IOClient() : _inner = new HttpClient();
 
   /// Sends an HTTP request and asynchronously returns the response.
   Future<StreamedResponse> send(BaseRequest request) {
     var stream = request.finalize();
 
     return Chain.track(_inner.openUrl(request.method, request.url))
         .then((ioRequest) {
+      var contentLength = request.contentLength == null ?
+          -1 : request.contentLength;
       ioRequest
           ..followRedirects = request.followRedirects
           ..maxRedirects = request.maxRedirects
-          ..contentLength = request.contentLength
+          ..contentLength = contentLength
           ..persistentConnection = request.persistentConnection;
       request.headers.forEach((name, value) {
         ioRequest.headers.set(name, value);
       });
       return Chain.track(stream.pipe(ioRequest));
     }).then((response) {
       var headers = {};
       response.headers.forEach((key, values) {
         headers[key] = values.join(',');
       });
 
+      var contentLength = response.contentLength == -1 ?
+          null : response.contentLength;
       return new StreamedResponse(
           response,
           response.statusCode,
-          response.contentLength,
+          contentLength: contentLength,
           request: request,
           headers: headers,
           isRedirect: response.isRedirect,
           persistentConnection: response.persistentConnection,
           reasonPhrase: response.reasonPhrase);
     });
   }
 
   /// Closes the client. This terminates all active connections. If a client
   /// remains unclosed, the Dart process may not terminate.
   void close() {
     if (_inner != null) _inner.close(force: true);
     _inner = null;
   }
 }