fix unsound cast failures in tests

tests/language_strong/many_named_arguments_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 
 class Fisk {
   method(
       {a: 'a',
       b: 'b',
@@ skipping 45 matching lines @@
         'v: $v, '
         'w: $w, '
         'x: $x, '
         'y: $y, '
         'z: $z';
   }
 }
 
 main() {
   var method = new Fisk().method;
-  var namedArguments = new Map();
+  var namedArguments = new Map<Symbol, dynamic>();
   namedArguments[const Symbol('a')] = 'a';
   Expect.stringEquals(
       EXPECTED_RESULT, Function.apply(method, [], namedArguments));
   Expect.stringEquals(
       EXPECTED_RESULT,
       new Fisk().method(
           a: 'a',
           b: 'b',
           c: 'c',
           d: 'd',
@@ skipping 40 matching lines @@
     'q: q, '
     'r: r, '
     's: s, '
     't: t, '
     'u: u, '
     'v: v, '
     'w: w, '
     'x: x, '
     'y: y, '
     'z: z';