Make Null a public class of dart:core.

runtime/lib/corelib_sources.gypi

 # Copyright (c) 2011, 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.
 
 # Sources visible via default library.
 
 {
   'sources': [
     'core_patch.dart',
     # The above file needs to be first as it imports required libraries.
@@ skipping 16 matching lines @@
     'identical.cc',
     'identical_patch.dart',
     'immutable_map.dart',
     'integers.cc',
     'integers.dart',
     'integers_patch.dart',
     'invocation_mirror.cc',
     'invocation_mirror.h',
     'invocation_mirror_patch.dart',
     'map_patch.dart',
+    'null_patch.dart',
     'object.cc',
     'object_patch.dart',
     'print_patch.dart',
     'regexp.cc',
     'regexp_jsc.cc',
     'regexp_jsc.h',
     'regexp_patch.dart',
     'stacktrace.cc',
     'stacktrace_patch.dart',
     'stopwatch_patch.dart',
     'stopwatch.cc',
     'string.cc',
     'string_patch.dart',
     'string_buffer_patch.dart',
     'type_patch.dart',
     'uri.cc',
     'uri_patch.dart',
     'weak_property.dart',
     'weak_property.cc',
   ],
 }