Stop supporting map literals with 1 type argument

pkg/http/test/utils.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 test_utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 52 matching lines @@
         var encoding = requiredEncodingForCharset(
             request.headers.contentType.charset);
         requestBody = decodeString(requestBodyBytes, encoding);
       } else {
         requestBody = requestBodyBytes;
       }
 
       var content = {
         'method': request.method,
         'path': request.path,
-        'headers': <String>{}
+        'headers': <String, String>{}
       };
       if (requestBody != null) content['body'] = requestBody;
       request.headers.forEach((name, values) {
         // These headers are automatically generated by dart:io, so we don't
         // want to test them here.
         if (name == 'cookie' || name == 'host') return;
 
         content['headers'][name] = values;
       });
 
@@ skipping 99 matching lines @@
 const isSocketIOException = const _SocketIOException();
 
 /// A matcher for functions that throw SocketIOException.
 const Matcher throwsSocketIOException =
     const Throws(isSocketIOException);
 
 class _SocketIOException extends TypeMatcher {
   const _SocketIOException() : super("SocketIOException");
   bool matches(item, MatchState matchState) => item is SocketIOException;
 }