Use an ES6 map for a linked identity hash map.

sdk/lib/_internal/compiler/js_lib/linked_hash_map.dart

 // Copyright (c) 2014, 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.
 
 // Efficient JavaScript based implementation of a linked hash map used as a
 // backing map for constant maps and the [LinkedHashMap] patch
 
 part of _js_helper;
 
 const _USE_ES6_MAPS = const bool.fromEnvironment("dart2js.use.es6.maps");
@@ skipping 26 matching lines @@
   static bool get _supportsEs6Maps {
     return JS('returns:bool;depends:none;effects:none;throws:never;gvn:true',
               'typeof Map != "undefined"');
   }
 
   JsLinkedHashMap();
 
   /// If ES6 Maps are available returns a linked hash-map backed by an ES6 Map.
   @ForceInline()
   factory JsLinkedHashMap.es6() {
-    return (_USE_ES6_MAPS  && JsLinkedHashMap._supportsEs6Maps)
+    return (_USE_ES6_MAPS && JsLinkedHashMap._supportsEs6Maps)
         ? new Es6LinkedHashMap<K, V>()
         : new JsLinkedHashMap<K, V>();
   }
 
   int get length => _length;
   bool get isEmpty => _length == 0;
   bool get isNotEmpty => !isEmpty;
 
   Iterable<K> get keys {
     return new LinkedHashMapKeyIterable<K>(this);
@@ skipping 158 matching lines @@
   V _removeHashTableEntry(var table, Object key) {
     if (table == null) return null;
     LinkedHashMapCell cell = _getTableEntry(table, key);
     if (cell == null) return null;
     _unlinkCell(cell);
     _deleteTableEntry(table, key);
     return cell.hashMapCellValue;
   }
 
   void _modified() {
-    // Value cycles after 2^30 modifications. If you keep hold of an
-    // iterator for that long, you might miss a modification
-    // detection, and iteration can go sour. Don't do that.
+    // Value cycles after 2^30 modifications so that modification counts are
+    // always unboxed (Smi) values. Modification detection will be missed if you
+    // make exactly some multiple of 2^30 modifications between advances of an
+    // iterator.
     _modifications = (_modifications + 1) & 0x3ffffff;
   }
 
   // Create a new cell and link it in as the last one in the list.
   LinkedHashMapCell _newLinkedCell(K key, V value) {
     LinkedHashMapCell cell = new LinkedHashMapCell(key, value);
     if (_first == null) {
       _first = _last = cell;
     } else {
       LinkedHashMapCell last = _last;
@@ skipping 177 matching lines @@
     } else if (_cell == null) {
       _current = null;
       return false;
     } else {
       _current = _cell.hashMapCellKey;
       _cell = _cell._next;
       return true;
     }
   }
 }