anchor.diff:    Backboard _backboard;
     Backboard get backboard => _backboard;
   
  +  final _propertyToField = HashMap<int, CoreFieldType>();
  +
     final List<Artboard> _artboards = [];
     List<Artboard> get artboards => _artboards;
   
  
positive.code:     var reader = BinaryReader(bytes);
      _header = RuntimeHeader.read(reader);
  
      _backboard = readRuntimeObject<Backboard>(reader);
      if (_backboard == null) {
        throw const RiveFormatErrorException(
            'expected first object to be a Backboard');
  
negative.code: 
              var numKeyedProperties = reader.readVarUint();
              for (int k = 0; k < numKeyedProperties; k++) {
                var keyedProperty = _readRuntimeObject<KeyedProperty>(reader);
                if (keyedProperty == null) {
                  continue;
                }
  