Reapply changes to Expando patching.

lib/coreimpl/corelib_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.
 
 {
   'sources': [
     'arrays.dart',
     'collections.dart',
     'date.dart',
     'dual_pivot_quicksort.dart',
     'duration_implementation.dart',
     'exceptions.dart',
-    'expando.dart',
     'future_implementation.dart',
     'hash_map_set.dart',
     'linked_hash_map.dart',
     'list.dart',
     'maps.dart',
     'options.dart',
     'print.dart',
     'queue.dart',
     'regexp.dart',
     'splay_tree.dart',
     'stopwatch_implementation.dart',
     'string.dart',
     'string_buffer.dart',
   ],
 }