add tool to check that deferred libraries contain the expected code

lib/json_info_codec.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.
 
 /// Converters and codecs for converting between JSON and [Info] classes.
 part of dart2js_info.info;
 
 // TODO(sigmund): add unit tests.
 class JsonToAllInfoConverter extends Converter<Map, AllInfo> {
   Map<String, Info> registry;
@@ skipping 30 matching lines @@
         var target = idMap[dep['id']];
         assert(target != null);
         src.uses.add(new DependencyInfo(target, dep['mask']));
       }
     });
 
     json['dependencies']?.forEach((k, deps) {
       result.dependencies[idMap[k]] = deps.map((d) => idMap[d]).toList();
     });
 
+    result.outputUnits.addAll(json['outputUnits'].map(parseOutputUnit));
+
     result.program = parseProgram(json['program']);
     // todo: version, etc
     return result;
   }
 
+  OutputUnitInfo parseOutputUnit(Map json) {
+    OutputUnitInfo result = parseId(json['id']);
+    result
+      ..name = json['name']
+      ..size = json['size'];
+    return result;
+  }
+
   LibraryInfo parseLibrary(Map json) {
     LibraryInfo result = parseId(json['id']);
     result
       ..name = json['name']
       ..uri = Uri.parse(json['canonicalUri'])
       ..outputUnit = parseId(json['outputUnit'])
       ..size = json['size'];
     for (var child in json['children'].map(parseId)) {
       if (child is FunctionInfo) {
         result.topLevelFunctions.add(child);
@@ skipping 340 matching lines @@
 
   visitTypedef(TypedefInfo info) => _visitBasicInfo(info)..['type'] = info.type;
 
   visitOutput(OutputUnitInfo info) => _visitBasicInfo(info);
 }
 
 class AllInfoJsonCodec extends Codec<AllInfo, Map> {
   final Converter<AllInfo, Map> encoder = new AllInfoToJsonConverter();
   final Converter<Map, AllInfo> decoder = new JsonToAllInfoConverter();
 }