Make LinkedHashMap also have a factory constructor and be customizable

sdk/lib/collection/hash_map.dart

 // Copyright (c) 2013, 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 dart.collection;
 
 /** Default function for equality comparison in customized HashMaps */
 bool _defaultEquals(a, b) => a == b;
 /** Default function for hash-code computation in customized HashMaps */
 int _defaultHashCode(a) => a.hashCode;
@@ skipping 23 matching lines @@
    * same as long as the map isn't changed.
    *
    * If [equals] is provided, it is used to compare the keys in the table with
    * new keys. If [equals] is omitted, the key's own [Object.operator==] is used
    * instead.
    *
    * Similar, if [hashCode] is provided, it is used to produce a hash value
    * for keys in order to place them in the hash table. If it is omitted, the
    * key's own [Object.hashCode] is used.
    *
+   * If using methods like [operator[]], [remove] and [containsKey] together
+   * with a custom equality and hashcode, an extra `isValidKey` function
+   * can be supplied. This function is called before calling [equals] or
+   * [hashCode] with an argument that may not be a [K] instance, and if the
+   * call returns false, the key is assumed to not be in the set.
+   * The [isValidKey] function defaults to just testing if the object is a
+   * [K] instance.
+   *
    * The used `equals` and `hashCode` method should always be consistent,
    * so that if `equals(a, b)` then `hashCode(a) == hashCode(b)`. The hash
    * of an object, or what it compares equal to, should not change while the
    * object is in the table. If it does change, the result is unpredictable.
    *
    * It is generally the case that if you supply one of [equals] and [hashCode],
    * you also want to supply the other. The only common exception is to pass
    * [identical] as the equality and use the default hash code.
    */
-  external factory HashMap({bool equals(K key1, K key2), int hashCode(K key)});
+  external factory HashMap({bool equals(K key1, K key2),
+                            int hashCode(K key),
+                            bool isValidKey(potentialKey)});
 
   /**
    * Creates a [HashMap] that contains all key value pairs of [other].
    */
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
   /**
    * Creates a [HashMap] where the keys and values are computed from the
@@ skipping 25 matching lines @@
    * overwrites the previous value.
    *
    * It is an error if the two [Iterable]s don't have the same length.
    */
   factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     HashMap<K, V> map = new HashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
 }