Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/hosted/resolve_constraints_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 @@
       "baz": "2.0.4"
     }).validate();
 
     d.packagesDir({
       "foo": "1.2.3",
       "bar": "2.3.4",
       "baz": "2.0.4"
     }).validate();
   });
 }