Require two type arguments for map literals (issue 4522).

tests/language/map_literal2_negative_test.dart

 // Copyright (c) 2011, 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.
 // VMOptions=--enable_type_checks
 //
 // Dart test program const map literals.
 
 class MapLiteral2NegativeTest<T> {
   test() {
     try {
-      var m = const <T>{"a": 0};  // Type parameter is not allowed with const.
+      var m = const <String, T>{"a": 0};  // Type parameter is not allowed with
+                                          // const.
     } catch (TypeError error) {
     }
   }
 }
 
 main() {
   var t = new MapLiteral2NegativeTest<int>();
   t.test();
 }