Stop supporting map literals with 1 type argument

pkg/http/lib/src/base_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 base_response;
 
 import 'dart:io';
 
 import 'base_request.dart';
 
@@ skipping 25 matching lines @@
   final bool isRedirect;
 
   /// Whether the server requested that a persistent connection be maintained.
   final bool persistentConnection;
 
   /// Creates a new HTTP response.
   BaseResponse(
       this.statusCode,
       this.contentLength,
       {this.request,
-       this.headers: const <String>{},
+       this.headers: const <String, String>{},
        this.isRedirect: false,
        this.persistentConnection: true,
        this.reasonPhrase});
 }