add tool to get breakdown of deferred libraries by size

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 41 matching lines @@
 /// - regexp: "dart:.*"
 ///
 /// # If your code lives under /my/project/dir, this will match any file loaded
 /// from a file:// url, and we use as a name the relative path to it.
 /// - regexp: "file:///my/project/dir/(.*)"
 ///```
 ///
 /// This example is very similar to [defaultGrouping].
 library dart2js_info.bin.library_size_split;
 
-import 'dart:convert';
 import 'dart:io';
 import 'dart:math' show max;
 
 import 'package:dart2js_info/info.dart';
+import 'package:dart2js_info/src/util.dart';
 import 'package:yaml/yaml.dart';
 
-main(args) {
+main(args) async {
   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 AllInfoJsonCodec().decode(json);
+  var info = await infoFromFile(args.first);
 
   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}/(.*)" }
 """;