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

pkg/serialization/lib/src/serialization_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): We should have an example and tests for subclassing
 // serialization rule rather than using the hard-coded ClosureToMap rule. And
 // possibly an abstract superclass that's designed to be subclassed that way.
 /**
@@ skipping 257 matching lines @@
   pullStateFrom(Iterator stream) {
     var dataLength = stream.next();
     var ruleData = new List();
     for (var i = 0; i < dataLength; i++) {
       ruleData.add(stream.next());
     }
     return ruleData;
   }
 }
 
-/** Helper function for PrimitiveRule to tell which objects it applies to. */
-bool isPrimitive(Object object) {
-  return object is num || object is String || object is bool;
-}
-
 /** Typedef for the object construction closure used in ClosureToMapRule. */
 typedef Object ConstructType(Map m);
 
 /** Typedef for the state-getting closure used in ClosureToMapRule. */
 typedef Map<String, Object> GetStateType(Object o);
 
 /** Typedef for the state-setting closure used in ClosureToMapRule. */
 typedef void NonEssentialStateType(Object o, Map m);
 
 /**
@@ skipping 65 matching lines @@
 class ClassMirrorRule extends SerializationRule {
   // TODO(alanknight): This probably generalizes to any named object.
   bool appliesTo(object) {
     return object is ClassMirror;
   }
   extractState(object, Function f) => f(object.simpleName);
   void flatten(object, Writer writer) {}
   inflateEssential(state, Reader r) => r.externalObjectNamed(state);
   inflateNonEssential(state, object, Reader r) {}
 }