Better ability to have hand-written custom rules and various cleanups.

pkg/serialization/lib/src/polyfill_identity_set.dart

 // 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.
 
 // TODO(alanknight): Replace with proper identity collection. Issue 4161
 library identity_set;
 
 import 'dart:collection';
 
 // Hash map implementation with open addressing and quadratic probing.
@@ skipping 285 matching lines @@
 
 /**
  * This class represents a pair of two objects, used by LinkedHashMap
  * to store a {key, value} in a list.
  */
 class _KeyValuePair<K, V> {
   _KeyValuePair(this.key, this.value) {}
 
   final K key;
   V value;
-}
-
+}