Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/git/check_out_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 '../../descriptor.dart' as d;
 import '../../test_pub.dart';
@@ skipping 20 matching lines @@
       ])
     ]).validate();
 
     d.dir(packagesPath, [
       d.dir('foo', [
         d.file('foo.dart', 'main() => "foo";')
       ])
     ]).validate();
   });
 }