Added key for completeness.

Author: awhitford

lib/src/widgets/grouped_checkbox.dart

@@ -181,6 +181,7 @@ class GroupedCheckbox<T> extends StatefulWidget {
   final ControlAffinity controlAffinity;
 
   GroupedCheckbox({
+    Key key,
     @required this.options,
     @required this.orientation,
     @required this.onChanged,
@@ -202,7 +203,7 @@ class GroupedCheckbox<T> extends StatefulWidget {
     this.wrapVerticalDirection = VerticalDirection.down,
     this.separator,
     this.controlAffinity = ControlAffinity.leading,
-  });
+  }) : super(key: key);
 
   @override
   _GroupedCheckboxState<T> createState() => _GroupedCheckboxState<T>();