Make the result of JSON.decode be marked by "null" in the reviver.

tests/lib/convert/codec2_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 'dart:convert';
 
 import 'package:expect/expect.dart';
 
 main() {
   final RAW = '["122ç",50,50,231]';
   final ENCODED = const [91, 34, 49, 50, 50, 195, 167, 34, 44,
                          53, 48, 44, 53, 48, 44, 50, 51, 49, 93];
   Expect.listEquals(ENCODED, UTF8.encode(RAW));
   Expect.equals(RAW, UTF8.decode(ENCODED));
 
   Expect.listEquals([], UTF8.encode(""));
   Expect.equals("", UTF8.decode([]));
 
   final JSON_ENCODED = RAW;
   Expect.equals(JSON_ENCODED, JSON.encode(["122ç", 50, 50, 231]));
   Expect.listEquals(["122ç", 50, 50, 231], JSON.decode(JSON_ENCODED));
 
   // Test that the reviver is passed to the decoder.
   var decoded = JSON.decode('{"p": 5}', reviver: (k, v) {
-    if (k == "") return v;
+    if (k == null) return v;
     return v * 2;
   });
   Expect.equals(10, decoded["p"]);
   var jsonWithReviver = new JsonCodec.withReviver((k, v) {
-    if (k == "") return v;
+    if (k == null) return v;
     return v * 2;
   });
   decoded = jsonWithReviver.decode('{"p": 5}');
   Expect.equals(10, decoded["p"]);
 
   // Test example from comments.
   final JSON_TO_BYTES = JSON.fuse(UTF8);
   List<int> bytes = JSON_TO_BYTES.encode(["json-object"]);
   decoded = JSON_TO_BYTES.decode(bytes);
   Expect.isTrue(decoded is List);
   Expect.equals("json-object", decoded[0]);
 }