add a JsonInfoCodec class

bin/code_deps.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 query for code dependencies. Currently this tool only
 /// supports the `some_path` query, which gives you the shortest path for how
 /// one function depends on another.
 ///
 /// You can run this tool as follows:
 /// ```bash
@@ skipping 31 matching lines @@
     exit(1);
   }
 
   var json;
   try {
     json = JSON.decode(new File(args[0]).readAsStringSync());
   } catch (e) {
     print('error: could not read ${args[0]}');
     exit(1);
   }
-  var info = new AllInfo.fromJson(json);
+  var info = new AllInfoJsonCodec().decode(json);
   var graph = graphFromInfo(info);
 
   var queryName = args[1];
   if (queryName == 'some_path') {
     if (args.length < 4) {
       print('missing arguments for `some_path`');
       exit(1);
     }
     var source = info.functions
         .firstWhere(_longNameMatcher(new RegExp(args[2])), orElse: () => null);
@@ skipping 57 matching lines @@
         if (seen.containsKey(neighbor)) continue;
         seen[neighbor] = node;
         queue.addLast(neighbor);
       }
     }
     return [];
   }
 }
 
 _longNameMatcher(RegExp regexp) => (e) => regexp.hasMatch(longName(e));