Stop working around issue 2644.

pkg/http/lib/src/base_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 base_client;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:typed_data';
 
@@ skipping 94 matching lines @@
   /// Sends a non-streaming [Request] and returns a non-streaming [Response].
   Future<Response> _sendUnstreamed(
       String method, url, Map<String, String> headers,
       [Map<String, String> fields]) {
     // Wrap everything in a Future block so that synchronous validation errors
     // are passed asynchronously through the Future chain.
     return async.then((_) {
       if (url is String) url = Uri.parse(url);
       var request = new Request(method, url);
 
-      if (headers != null) mapAddAll(request.headers, headers);
+      if (headers != null) request.headers.addAll(headers);
       if (fields != null && !fields.isEmpty) request.bodyFields = fields;
 
       return send(request);
     }).then(Response.fromStream);
   }
 
   /// Throws an error if [response] is not successful.
   void _checkResponseSuccess(url, Response response) {
     if (response.statusCode < 400) return;
     var message = "Request to $url failed with status ${response.statusCode}";
     if (response.reasonPhrase != null) {
       message = "$message: ${response.reasonPhrase}";
     }
     throw new HttpException("$message.");
   }
 
   /// Closes the client and cleans up any resources associated with it. It's
   /// important to close each client when it's done being used; failing to do so
   /// can cause the Dart process to hang.
   void close() {}
 }