Make pubspec.lock keep relative path if it is originally specified as relative in pubspec.yaml (iss…

sdk/lib/_internal/pub/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 'package:unittest/unittest.dart';
 import 'package:yaml/yaml.dart';
 
 import '../lib/src/lock_file.dart';
@@ skipping 161 matching lines @@
       setUp(() {
         lockfile = new LockFile.empty();
       });
 
       test('dumps the lockfile to YAML', () {
         lockfile.packages['foo'] = new PackageId(
             'foo', mockSource.name, new Version.parse('1.2.3'), 'foo desc');
         lockfile.packages['bar'] = new PackageId(
             'bar', mockSource.name, new Version.parse('3.2.1'), 'bar desc');
 
-        expect(loadYaml(lockfile.serialize()), equals({
+        expect(loadYaml(lockfile.serialize(null, sources)), equals({
           'packages': {
             'foo': {
               'version': '1.2.3',
               'source': 'mock',
               'description': 'foo desc'
             },
             'bar': {
               'version': '3.2.1',
               'source': 'mock',
               'description': 'bar desc'
             }
           }
         }));
       });
     });
   });
 }