Fix lock_file_test.

test/lock_file_test.dart

 // Copyright (c) 2012, 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 lock_file_test;
 
 import 'dart:async';
 
 import 'package:pub/src/lock_file.dart';
 import 'package:pub/src/package.dart';
@@ skipping 193 matching lines @@
 
     test('serialize() dumps the lockfile to YAML', () {
       var lockfile = new LockFile([
         new PackageId(
           'foo', mockSource.name, new Version.parse('1.2.3'), 'foo desc'),
         new PackageId(
           'bar', mockSource.name, new Version.parse('3.2.1'), 'bar desc')
       ], sources);
 
       expect(loadYaml(lockfile.serialize(null)), equals({
+        'sdk': 'any',
         'packages': {
           'foo': {
             'version': '1.2.3',
             'source': 'mock',
             'description': 'foo desc'
           },
           'bar': {
             'version': '3.2.1',
             'source': 'mock',
             'description': 'bar desc'
           }
         }
       }));
     });
   });
 }