Revert "Add custom equals and hashCode for HashMap implementation."

sdk/lib/collection/hash_map.dart

Index: sdk/lib/collection/hash_map.dart
diff --git a/sdk/lib/collection/hash_map.dart b/sdk/lib/collection/hash_map.dart
index 4481d85cb86653d16a767f5acbe3ed5a6e8d1b31..f9b7d45c8a32f1e5f0cd76d97ac35d653e4c512f 100644
--- a/sdk/lib/collection/hash_map.dart
+++ b/sdk/lib/collection/hash_map.dart
@@ -20,7 +20,7 @@ int _defaultHashCode(a) => a.hashCode;
  *
  * The map allows `null` as a key.
  */
-abstract class HashMap<K, V> implements Map<K, V> {
+class HashMap<K, V> implements Map<K, V> {
   /**
    * Creates a hash-table based [Map].
    *
@@ -40,12 +40,22 @@ abstract class HashMap<K, V> implements Map<K, V> {
    * 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)});
+  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].
@@ -90,4 +100,28 @@ abstract class HashMap<K, V> implements Map<K, V> {
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
+
+  external int get length;
+  external bool get isEmpty;
+  external bool get isNotEmpty;
+
+  external Iterable<K> get keys;
+  external Iterable<V> get values;
+
+  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));
+
+  String toString() => Maps.mapToString(this);
 }