Make HashMap constructor a factory.

pkg/serialization/lib/src/serialization_helpers.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.
 
 /**
  * This contains extra functions and classes useful for implementing
  * serialiation. Some or all of these will be removed once the functionality is
  * available in the core library.
  */
 library serialization_helpers;
@@ skipping 191 matching lines @@
   get hashCode => _value.hashCode;
   get object => _value;
 }
 
 /**
  * This provides an identity map. We wrap all the objects in
  * an [_IdentityMapKey] that compares using the identity of the
  * wrapped objects. It also treats equal primitive values as identical
  * to conserve space.
  */
-class IdentityMap<K, V> extends HashMap<K, V> {
+class IdentityMap<K, V> extends LinkedHashMap<K, V> {
 // TODO(alanknight): Replace with a system identity-based map once
 // one is available. Issue 4161.
+// TODO(lrn): Replace with identity map when custom hash maps are introduced
+// (which is soon).
 
   // Check before wrapping because some methods may call others, e.g. on
   // dart2js putIfAbsent calls containsKey, so without this we wrap forever.
   _wrap(Object key) =>
       (key is _IdentityMapKey) ? key : new _IdentityMapKey(key);
   _unwrap(_IdentityMapKey wrapper) => wrapper.object;
 
   Iterable<K> get keys => super.keys.map((x) => _unwrap(x));
   Iterable<V> get values => super.values;
 
   void forEach(void f(K key, V value)) {
     super.forEach((k, v) => f(_unwrap(k), v));
   }
 
   V operator [](K key) => super[_wrap(key)];
 
   void operator []=(K key, V value) {
       super[_wrap(key)] = value;
   }
 
   V putIfAbsent(K key, Function ifAbsent) =>
       super.putIfAbsent(_wrap(key), ifAbsent);
 
   bool containsKey(Object key) => super.containsKey(_wrap(key));
 
   V remove(Object key) => super.remove(_wrap(key));
 }