anchor.diff:      _object = obj;
       if (obj == null) {
         _mode = _MODE_IDENTITY_;
  -    } else if (field == null) {
  +      return;
  +    }
  +
  +    if (field == null) {
         _instanceMirror = null;
         if (obj is Map) {
           if (_mode != _MODE_MAP_) {
  
positive.code:     identical(_iterable, collection)) {
        // Short circuit and assume that the list has not been modified.
        return false;
      } else if (collection is List) {
        List list = collection;
        for(int index = 0; index < list.length; index++) {
          var item = list[index];
  
negative.code:       } else {
          _mode = _MODE_IDENTITY_;
        }
  
        return;
      }
  
      if (obj is Map) {
        _mode =  _MODE_MAP_FIELD_;
        _instanceMirror = null;
      } else if (_getter != null) {
        _mode = _MODE_GETTER_;
        _instanceMirror = null;
      } else {
        _mode = _MODE_REFLECT_;
        _instanceMirror = reflect(obj);
      }
    }
  
  