Stop supporting map literals with 1 type argument

pkg/http/lib/src/multipart_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 multipart_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 import 'dart:uri';
@@ skipping 64 matching lines @@
   /// are written to this sink at that point will be used.
   Sink<MultipartFile> get files => _files;
 
   /// The private version of [files], typed so that the underlying collection is
   /// accessible.
   final CollectionSink<MultipartFile> _files;
 
   /// Creates a new [MultipartRequest].
   MultipartRequest(String method, Uri url)
     : super(method, url),
-      fields = <String>{},
+      fields = {},
       _files = new CollectionSink<MultipartFile>(<MultipartFile>[]);
 
   /// Freezes all mutable fields and returns a single-subscription [ByteStream]
   /// that will emit the request body.
   ByteStream finalize() {
     // TODO(nweiz): freeze fields and files
     var boundary = _boundaryString(_BOUNDARY_LENGTH);
     headers['content-type'] = 'multipart/form-data; boundary="$boundary"';
     headers['content-transfer-encoding'] = 'binary';
     super.finalize();
@@ skipping 70 matching lines @@
   String _boundaryString(int length) {
     var prefix = "dart-http-boundary-";
     var list = new List<int>.fixedLength(length - prefix.length);
     for (var i = 0; i < list.length; i++) {
       list[i] = _BOUNDARY_CHARACTERS[
           _random.nextInt(_BOUNDARY_CHARACTERS.length)];
     }
     return "$prefix${new String.fromCharCodes(list)}";
   }
 }