Make LinkedHashMap also have a factory constructor and be customizable

sdk/lib/collection/linked_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;
 
 /**
  * A hash-table based implementation of [Map].
  *
  * Keys insertion order is remembered, and keys are iterated in insertion order.
  * Values are iterated in their corresponding key's order.
  *
  * The keys of a `LinkedHashMap` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the keys (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
  *
  * The map allows `null` as a key.
  */
-class LinkedHashMap<K, V> implements HashMap<K, V> {
-  external LinkedHashMap();
+abstract class LinkedHashMap<K, V> implements HashMap<K, V> {
+  external factory LinkedHashMap({ bool equals(K key1, K key2),
+                                   int hashCode(K key),
+                                   bool isValidKey(potentialKey) });
 
   /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
    */
   factory LinkedHashMap.from(Map<K, V> other) {
     return new LinkedHashMap<K, V>()..addAll(other);
   }
 
   /**
    * Creates a [LinkedHashMap] where the keys and values are computed from the
@@ skipping 24 matching lines @@
    * If [keys] contains the same object multiple times, the last occurrence
    * overwrites the previous value.
    *
    * It is an error if the two [Iterable]s don't have the same length.
    */
   factory LinkedHashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     LinkedHashMap<K, V> map = new LinkedHashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
-
-  external bool containsKey(Object key);
-
-  external bool containsValue(Object value);
-
-  external void addAll(Map<K, V> other);
-
-  external V operator [](Object key);
-
-  external void operator []=(K key, V value);
-
-  external V putIfAbsent(K key, V ifAbsent());
-
-  external V remove(Object key);
-
-  external void clear();
-
-  external void forEach(void action (K key, V value));
-
-  /** The keys of the map, in insertion order. */
-  external Iterable<K> get keys;
-  /** The values of the map, in the order of their corresponding [keys].*/
-  external Iterable<V> get values;
-
-  external int get length;
-
-  external bool get isEmpty;
-
-  external bool get isNotEmpty;
-
-  String toString() => Maps.mapToString(this);
 }