Stop supporting map literals with 1 type argument

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 'base_client.dart';
@@ skipping 38 matching lines @@
       request.headers.forEach((name, value) {
         underlyingRequest.headers.set(name, value);
       });
 
       chainToCompleter(
           stream.pipe(wrapOutputStream(underlyingRequest.outputStream)),
           pipeCompleter);
     };
 
     connection.onResponse = (response) {
-      var headers = <String>{};
+      var headers = <String, String>{};
       response.headers.forEach((key, value) => headers[key] = value);
 
       if (completed) return;
 
       completed = true;
       completer.complete(new StreamedResponse(
           wrapInputStream(response.inputStream),
           response.statusCode,
           response.contentLength,
           request: request,
           headers: headers,
           isRedirect: response.isRedirect,
           persistentConnection: response.persistentConnection,
           reasonPhrase: response.reasonPhrase));
     };
 
     return Future.wait([
       completer.future,
       pipeCompleter.future
     ]).then((values) => values.first);
   }
 
   /// 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.shutdown(force: true);
     _inner = null;
   }
 }