Added an extra deprecated annotation so that users are alerted to the deprecation of label. (I noticed that my code was not alerting with the deprecation only on the field declaration.)

Author: awhitford

lib/src/form_builder_field_option.dart

@@ -8,10 +8,15 @@ class FormBuilderFieldOption extends StatelessWidget {
 
   const FormBuilderFieldOption({
     Key key,
-    this.label,
-    @required this.value,
+    @Deprecated('Use `child` instead. Will be removed in the next major version.')
+        // ignore: deprecated_member_use_from_same_package
+        this.label,
+    @required
+        this.value,
     this.child,
-  })  : assert(label == null || child == null),
+  })  :
+        // ignore: deprecated_member_use_from_same_package
+        assert(label == null || child == null),
         super(key: key);
 
   @override