add a JsonInfoCodec class

test/all_test.dart

 // Copyright (c) 2015, 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:dart2js_info/info.dart';
 import 'package:test/test.dart';
 
 main() {
   // TODO(sigmund): add more tests
   group('parse', () {
     test('empty', () {
       var json = {
         'elements': {
           'library': {},
           'class': {},
           'function': {},
           'field': {},
           'typedef': {},
         },
         'holding': {},
         'program': {'size': 10},
       };
 
-      expect(new AllInfo.fromJson(json).program.size, 10);
+      expect(new AllInfoJsonCodec().decode(json).program.size, 10);
     });
   });
 }