Make HashMap constructor a factory.

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;
+
 /**
  * A hash-table based implementation of [Map].
  *
  * The keys of a `HashMap` 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 HashMap<K, V> implements Map<K, V> {
-  external HashMap();
+  /**
+   * Creates an unordered hash-table based [Map].

[floitsch, 2013/09/03 11:19:49]

I'm not sure I like the word "unordered" here.
Don't put it in the main-description, but explain below that the keys have no
order.

[Lasse Reichstein Nielsen, 2013/09/03 11:44:28]

Done.

+   *
+   * 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.
+   *
+   * 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.
+   */
+  factory HashMap({bool equals(K key1, K key2), int hashCode(K key)}) {
+    if (equals != null || hashCode != null) {
+      if (equals == null) {
+        equals = _defaultEquals;
+      } else if (hashCode == null) {
+        hashCode = _defaultHashCode;
+      }
+      // Create new CustomHashMap<K, V>(equals, hashCode).
+      throw new UnimplementedError("Not implemented yet");
+    }
+    return new HashMap<K, V>._internal();
+  }
+
+  /** Creates an empty `HashMap` with the default equals and hashCode. */
+  external HashMap._internal();
 
   /**
    * 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 49 matching lines @@
 
   external V putIfAbsent(K key, V ifAbsent());
 
   external V remove(Object key);
   external void clear();
 
   external void forEach(void action(K key, V value));
 
   String toString() => Maps.mapToString(this);
 }