Add Null class to dart:core.

sdk/lib/core/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': [
     'core.dart',
     # The above file needs to be first as it lists the parts below.
     'bool.dart',
     'comparable.dart',
     'date_time.dart',
     'double.dart',
     'duration.dart',
     'errors.dart',
     'exceptions.dart',
     'expando.dart',
     'function.dart',
     'identical.dart',
     'int.dart',
     'invocation.dart',
     'iterable.dart',
     'iterator.dart',
     'list.dart',
     'map.dart',
+    'null.dart',
     'num.dart',
     'object.dart',
     'pattern.dart',
     'print.dart',
     'regexp.dart',
     'set.dart',
     'stacktrace.dart',
     'stopwatch.dart',
     'string.dart',
     'string_buffer.dart',
     'string_sink.dart',
     'symbol.dart',
     'type.dart',
     'uri.dart',
   ],
 }