add a JsonInfoCodec class

bin/debug_info.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.
 
 /// Tool used mainly by dart2js developers to debug the generated info and check
 /// that it is consistent and that it covers all the data we expect it to cover.
 library dart2js_info.bin.debug_info;
 
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:dart2js_info/info.dart';
 import 'package:dart2js_info/src/graph.dart';
 
 main(args) {
   if (args.length < 1) {
     print('usage: dart tool/debug_info.dart path-to-info.json '
         '[--show-library libname]');
     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 debugLibName;
 
   if (args.length > 2 && args[1] == '--show-library') {
     debugLibName = args[2];
   }
 
   // Gather data from visiting all info elements.
   var tracker = new _SizeTracker(debugLibName);
   info.accept(tracker);
 
@@ skipping 218 matching lines @@
     _pass('dependency data is consistent');
   } else {
     _fail('inconsistencies in dependency data:\n'
         '   $inUsesNotInDependencies edges missing from "dependencies" graph\n'
         '   $inDependenciesNotInUses edges missing from "uses" graph');
   }
 }
 
 _pass(String msg) => print('\x1b[32mPASS\x1b[0m: $msg');
 _fail(String msg) => print('\x1b[31mFAIL\x1b[0m: $msg');