Use JsonMap even for JSON.decode with custom reviver.

tests/corelib/json_map_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.
 
 library json_map_test;
 
 import "package:expect/expect.dart";
 import 'dart:convert' show JSON;
 import 'dart:collection' show LinkedHashMap, HashMap;
 
-Map jsonify(Map map) => JSON.decode(JSON.encode(map));
+bool useReviver = false;
+Map jsonify(Map map) {
+  String encoded = JSON.encode(map);
+  return useReviver
+      ? JSON.decode(encoded, reviver: (key, value) => value)
+      : JSON.decode(encoded);
+}
 
 List listEach(Map map) {
   var result = [];
   map.forEach((String key, value) {
     result.add(key);
     result.add(value);
   });
   return result;
 }
 
 void main() {
+  test(false);
+  test(true);
+}
+
+void test(bool revive) {
+  useReviver = revive;
   testEmpty(jsonify({}));
   testAtoB(jsonify({'a': 'b'}));
 
   Map map = jsonify({});
   map['a'] = 'b';
   testAtoB(map);
 
   map = jsonify({});
   Expect.equals('b', map.putIfAbsent('a', () => 'b'));
   testAtoB(map);
@@ skipping 282 matching lines @@
 }
 
 void testListEntry() {
   Map map = jsonify({'a': [7, 8, {'b': 9}]});
   List list = map['a'];
   Expect.equals(3, list.length);
   Expect.equals(7, list[0]);
   Expect.equals(8, list[1]);
   Expect.equals(9, list[2]['b']);
 }