anchor.diff:  
     // Number of deleted keys.
     int _deletedKeys = 0;
  +
  +  // Note: All fields are initialized in a single constructor so that the VM
  +  // recognizes they cannot hold null values. This makes a big (20%) performance
  +  // difference on some operations.
  +  _HashFieldBase(int dataSize) : this._data = new List(dataSize);
   }
   
   // Base class for VM-internal classes; keep in sync with _HashFieldBase.
  
positive.code:     with _HashBase, _OperatorEqualsAndHashCode, SetMixin<E>
      implements LinkedHashSet<E> {
  
    _CompactLinkedHashSet() {
      assert(_HashBase._UNUSED_PAIR == 0);
      _index = new Uint32List(_HashBase._INITIAL_INDEX_SIZE);
      _hashMask = _HashBase._indexSizeToHashMask(_HashBase._INITIAL_INDEX_SIZE);
      _data = new List(_HashBase._INITIAL_INDEX_SIZE >> 1);
    }
  
    int get length => _usedData - _deletedKeys;
  
negative.code:   V remove(Object o) => _validKey(o) ? super.remove(o) : null;
  
    _CompactLinkedCustomHashMap(this._equality, this._hasher, validKey)
        : _validKey = (validKey != null) ? validKey : new _TypeTest<K>().test,
          super(_HashBase._INITIAL_INDEX_SIZE);
  }
  
  // Iterates through _data[_offset + _step], _data[_offset + 2*_step], ...
  