Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/lish/preview_package_validation_has_no_warnings_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:json' as json;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
 
 import '../descriptor.dart' as d;
@@ skipping 10 matching lines @@
     d.dir(appPath, [d.pubspec(pkg)]).create();
 
     var server = new ScheduledServer();
     var pub = startPublish(server, args: ['--dry-run']);
 
     pub.shouldExit(0);
     expect(pub.remainingStderr(),
         completion(contains('Package has 0 warnings.')));
   });
 }