add tool to get breakdown of deferred libraries by size

lib/src/util.dart

Index: lib/src/util.dart
diff --git a/lib/src/util.dart b/lib/src/util.dart
index 1b8a0847b2373b6899b3a791db9c428289b01bd4..b00f4347ab0eb9dab935595c6aa3c4f17f830b0a 100644
--- a/lib/src/util.dart
+++ b/lib/src/util.dart
@@ -4,7 +4,12 @@
 
 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].
@@ -124,3 +129,8 @@ String recursiveDiagnosticString(Measurements measurements, Metric metric) {
   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));
+}