Get rid of the polyfill identity set implementation in favour of a

pkg/serialization/lib/src/basic_rule.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.
 
 part of serialization;
 
 // TODO(alanknight): Figure out how to reasonably separate out the things
 // that require reflection without making the API more awkward. Or if that is
 // in fact necessary. Maybe the tree-shaking will just remove it if unused.
 
@@ skipping 81 matching lines @@
    * Note that the function is passed the owning object as well as the field
    * value, but that it is passed as a mirror.
    */
   specialTreatmentFor(String fieldName, SetWithFunction setWith) {
     fields.addAllByName([fieldName]);
     _Field field = fields.named(fieldName);
     Function setter = (setWith == null) ? field.defaultSetter : setWith;
     field.specialTreatment = setter;
   }
 
-  // TODO(alanknight): Polyfill for the non-hashability of mirrors. Issue 6880.
-  get typeWrapped => new ClassMirrorWrapper(type);
-
   /** Return the name of the constructor used to create new instances on read.*/
   String get constructorName => constructor.name;
 
   /** Return the list of field names to be passed to the constructor.*/
   List<String> get constructorFields => fields.constructorFieldNames();
 
   /** Return the list of field names not used in the constructor. */
   List<String> get regularFields => fields.regularFieldNames();
 
   String toString() => "Basic Rule for ${type.simpleName}";
@@ skipping 428 matching lines @@
   List fieldList;
   _MapWrapper(this.fieldList);
   _MapWrapper.fromMap(this._map, this.fieldList);
 
   operator [](key) => _map[fieldList[key].name];
   operator []=(key, value) { _map[fieldList[key].name] = value; }
   get length => _map.length;
 
   asMap() => _map;
 }