Add a new implementation of HashMap that uses JS objects for its (multiple) hash tables.

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;
 
 class _HashMapTable<K, V> extends _HashTable<K> {
   static const int _INITIAL_CAPACITY = 8;
   static const int _VALUE_INDEX = 1;
 
   _HashMapTable() : super(_INITIAL_CAPACITY);
 
   int get _entrySize => 2;
 
   V _value(int offset) => _table[offset + _VALUE_INDEX];
   void _setValue(int offset, V value) { _table[offset + _VALUE_INDEX] = value; }
 
   _copyEntry(List fromTable, int fromOffset, int toOffset) {
     _table[toOffset + _VALUE_INDEX] = fromTable[fromOffset + _VALUE_INDEX];
   }
 }
 
 class HashMap<K, V> implements Map<K, V> {
-  final _HashMapTable<K, V> _hashTable;
+  final _HashMapTable<K, V> _hashTable = new _HashMapTable<K, V>();
 
-  HashMap() : _hashTable = new _HashMapTable<K, V>() {
-    _hashTable._container = this;
-  }
+  external factory HashMap();

[Ivan Posva, 2013/03/22 16:32:47]

Why stop here? There are plenty of other opportunities below. The way it is
structured even after your half-way ripping it apart is that you need to have
this internal _hashTable object even if you did not really need it.

[kasperl, 2013/03/22 16:36:01]

I need to have this _internal hashTable object even when I don't really need it?
I don't understand that?

 
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
+  HashMap._internal() {
+    _hashTable._container = this;
+  }
+
   bool containsKey(K key) {
     return _hashTable._get(key) >= 0;
   }
 
   bool containsValue(V value) {
     List table = _hashTable._table;
     int entrySize = _hashTable._entrySize;
     for (int offset = 0; offset < table.length; offset += entrySize) {
       if (!_hashTable._isFree(table[offset]) &&
           _hashTable._value(offset) == value) {
@@ skipping 83 matching lines @@
   Iterable<K> get keys => new _HashTableKeyIterable<K>(_hashTable);
   Iterable<V> get values =>
       new _HashTableValueIterable<V>(_hashTable, _HashMapTable._VALUE_INDEX);
 
   int get length => _hashTable._elementCount;
 
   bool get isEmpty => _hashTable._elementCount == 0;
 
   String toString() => Maps.mapToString(this);
 }