Add SetBase/SetMixin.

sdk/lib/collection/collection_sources.gypi

 # 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.
 
 # This file contains all sources for the dart:collection library.
 {
   'sources': [
     'collection.dart',
     # The above file needs to be first as it lists the parts below.
     'collections.dart',
     'hash_map.dart',
     'hash_set.dart',
     'iterable.dart',
     'iterator.dart',
     'linked_hash_map.dart',
     'linked_hash_set.dart',
     'linked_list.dart',
     'list.dart',
     'maps.dart',
     'queue.dart',
+    'set.dart',
     'splay_tree.dart',
   ],
 }