Update unittest to new package layout.

utils/tests/pub/pub_update_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('pub_tests');
 
 #import('dart:io');
 
 #import('test_pub.dart');
-#import('../../../pkg/unittest/unittest.dart');
+#import('../../../pkg/unittest/lib/unittest.dart');
 
 main() {
   group('requires', () {
     test('a pubspec', () {
       dir(appPath, []).scheduleCreate();
 
       schedulePub(args: ['update'],
           error: const RegExp(@'^Could not find a file named "pubspec.yaml"'),
           exitCode: 1);
 
@@ skipping 67 matching lines @@
     schedulePub(args: ['update'],
         output: '''
         Dependencies updated!
         ''');
 
     packagesDir({"foo": null}).scheduleValidate();
 
     run();
   });
 }