Convert null-marker in HashMap.forEach.

runtime/lib/collection_patch.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.
 
 patch class HashMap<K, V> {
   final _HashMapTable<K, V> _hashTable = new _HashMapTable<K, V>();
 
   /* patch */ HashMap() {
     _hashTable._container = this;
   }
@@ skipping 76 matching lines @@
     _hashTable._clear();
   }
 
   /* patch */ void forEach(void action(K key, V value)) {
     int modificationCount = _hashTable._modificationCount;
     List table = _hashTable._table;
     int entrySize = _hashTable._entrySize;
     for (int offset = 0; offset < table.length; offset += entrySize) {
       Object entry = table[offset];
       if (!_hashTable._isFree(entry)) {
-        K key = entry;
+        K key = identical(entry, _NULL) ? null : entry;
         V value = _hashTable._value(offset);
         action(key, value);
         _hashTable._checkModification(modificationCount);
       }
     }
   }
 
   /* patch */ Iterable<K> get keys => new _HashTableKeyIterable<K>(_hashTable);
   /* patch */ Iterable<V> get values =>
       new _HashTableValueIterable<V>(_hashTable, _HashMapTable._VALUE_INDEX);
@@ skipping 893 matching lines @@
 
   _LinkedHashMapTable() : super(_INITIAL_CAPACITY);
 
   V _value(int offset) => _table[offset + _VALUE_INDEX];
   void _setValue(int offset, V value) { _table[offset + _VALUE_INDEX] = value; }
 
   _copyEntry(List oldTable, int fromOffset, int toOffset) {
     _table[toOffset + _VALUE_INDEX] = oldTable[fromOffset + _VALUE_INDEX];
   }
 }