add a JsonInfoCodec class

bin/library_size_split.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.
 
 /// Command-line tool to show the size distribution of generated code among
 /// libraries. Libraries can be grouped using regular expressions. You can
 /// specify what regular expressions to use by providing a `grouping.yaml` file.
 /// The format of the `grouping.yaml` file is as follows:
 /// ```yaml
 /// groups:
@@ skipping 57 matching lines @@
 
 main(args) {
   if (args.length < 1) {
     print('usage: dart tool/library_size_split.dart '
         'path-to-info.json [grouping.yaml]');
     exit(1);
   }
 
   var filename = args[0];
   var json = JSON.decode(new File(filename).readAsStringSync());
-  var info = new AllInfo.fromJson(json);
+  var info = new AllInfoJsonCodec().decode(json);
 
   var groupingText =
       args.length > 1 ? new File(args[1]).readAsStringSync() : defaultGrouping;
   var groupingYaml = loadYaml(groupingText);
   var groups = [];
   for (var group in groupingYaml['groups']) {
     groups.add(new _Group(
         group['name'], new RegExp(group['regexp']), group['cluster'] ?? 0));
   }
 
@@ skipping 115 matching lines @@
 - { name: "Core libs", regexp: "dart:.*", cluster: 2}
 # We omitted `name` to extract the group name from the regexp directly.
 # Here the name is the name of the package:
 - { regexp: "package:([^/]*)", cluster: 1}
 # Here the name is the url of the package and dart core libraries:
 - { regexp: "package:.*"}
 - { regexp: "dart:.*"}
 # Here the name is the relative path of loose files:
 - { regexp: "file://${Directory.current.path}/(.*)" }
 """;