add tool to get breakdown of deferred libraries by size

lib/src/util.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.
 
 library dart2js_info.src.util;
 
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
 import 'package:dart2js_info/info.dart';
+
 import 'graph.dart';
 
 /// Computes a graph of dependencies from [info].
 Graph<Info> graphFromInfo(AllInfo info) {
   print('  info: dependency graph information is work in progress and'
       ' might be incomplete');
   // Note: we are combining dependency information that is computed in two ways
   // (functionInfo.uses vs allInfo.dependencies).
   // TODO(sigmund): fix inconsistencies between these two ways, stick with one
   // of them.
@@ skipping 99 matching lines @@
       } else {
         sb.write(' + ');
       }
       helper(sub);
     }
     sb.write(')');
   }
   helper(metric);
   return sb.toString();
 }
+
+Future<AllInfo> infoFromFile(String fileName) async {
+  var file = await new File(fileName).readAsString();
+  return new AllInfoJsonCodec().decode(JSON.decode(file));
+}