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

pkg/http/lib/src/base_request.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 base_request;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'byte_stream.dart';
 import 'client.dart';
 import 'streamed_response.dart';
 import 'utils.dart';
 
 /// The base class for HTTP requests.
 ///
 /// Subclasses of [BaseRequest] can be constructed manually and passed to
 /// [BaseClient.send], which allows the user to provide fine-grained control
 /// over the request properties. However, usually it's easier to use convenience
 /// methods like [get] or [BaseClient.get].
 abstract class BaseRequest {
   /// The HTTP method of the request. Most commonly "GET" or "POST", less
   /// commonly "HEAD", "PUT", or "DELETE". Non-standard method names are also
   /// supported.
   final String method;
 
   /// The URL to which the request will be sent.
   final Uri url;
 
-  /// The size of the request body, in bytes. This defaults to -1, which
-  /// indicates that the size of the request is not known in advance.
+  /// The size of the request body, in bytes.
+  ///
+  /// This defaults to `null`, which indicates that the size of the request is
+  /// not known in advance.
   int get contentLength => _contentLength;
-  int _contentLength = -1;
+  int _contentLength;
 
   set contentLength(int value) {
+    if (value != null && value < 0) {
+      throw new ArgumentError("Invalid content length $value.");
+    }
     _checkFinalized();
     _contentLength = value;
   }
 
   /// Whether a persistent connection should be maintained with the server.
   /// Defaults to true.
   bool get persistentConnection => _persistentConnection;
   bool _persistentConnection = true;
 
   set persistentConnection(bool value) {
@@ skipping 57 matching lines @@
   /// Sends this request.
   ///
   /// This automatically initializes a new [Client] and closes that client once
   /// the request is complete. If you're planning on making multiple requests to
   /// the same server, you should use a single [Client] for all of those
   /// requests.
   Future<StreamedResponse> send() {
     var client = new Client();
     return client.send(this).then((response) {
       var stream = onDone(response.stream, client.close);
+      var contentLength = response.contentLength < 0 ?
+          null : response.contentLength;
       return new StreamedResponse(
           new ByteStream(stream),
           response.statusCode,
-          response.contentLength,
+          contentLength: contentLength,
           request: response.request,
           headers: response.headers,
           isRedirect: response.isRedirect,
           persistentConnection: response.persistentConnection,
           reasonPhrase: response.reasonPhrase);
     }).catchError((e) {
       client.close();
       throw e;
     });
   }
 
   // Throws an error if this request has been finalized.
   void _checkFinalized() {
     if (!finalized) return;
     throw new StateError("Can't modify a finalized Request.");
   }
 
   String toString() => "$method $url";
 }