Expose the WeakProperty type.

runtime/lib/lib_impl_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.
 
 # Implementation sources.
 
 {
   'sources': [
     'array.cc',
     'array.dart',
     'bool.dart',
     'date.cc',
     'date_patch.dart',
     'double.cc',
     'double.dart',
     'growable_array.cc',
     'growable_array.dart',
     'immutable_map.dart',
     'integers.cc',
     'integers.dart',
     'math.dart',
     'math.cc',
     'regexp.cc',
     'regexp_jsc.cc',
     'regexp_jsc.h',
     'regexp.dart',
     'stopwatch_patch.dart',
     'string.cc',
     'string.dart',
+    'weak_property.cc',
+    'weak_property.dart',
   ],
 }