Stop supporting map literals with 1 type argument

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:io';
 import 'dart:isolate';
 import 'dart:uri';
@@ skipping 64 matching lines @@
   // TODO(nweiz): make this a HttpHeaders object
   /// The headers for this request.
   final Map<String, String> headers;
 
   /// Whether the request has been finalized.
   bool get finalized => _finalized;
   bool _finalized = false;
 
   /// Creates a new HTTP request.
   BaseRequest(this.method, this.url)
-    : headers = <String>{};
+    : headers = <String, String>{};
 
   /// Finalizes the HTTP request in preparation for it being sent. This freezes
   /// all mutable fields and returns a single-subscription [ByteStream] that
   /// emits the body of the request.
   ///
   /// The base implementation of this returns null rather than a [ByteStream];
   /// subclasses are responsible for creating the return value, which should be
   /// single-subscription to ensure that no data is dropped. They should also
   /// freeze any additional mutable fields they add that don't make sense to
   /// change after the request headers are sent.
@@ skipping 30 matching lines @@
   }
 
   /// 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";
 }