[roll] Fixes for Flutter roll

Change-Id: I89bcf6d532e8b105edf6c531867b4aed0e5d4b84
diff --git a/public/dart/fuchsia_scenic_flutter/lib/src/child_view_render_box.dart b/public/dart/fuchsia_scenic_flutter/lib/src/child_view_render_box.dart
index 2b8660b..e1a4968 100644
--- a/public/dart/fuchsia_scenic_flutter/lib/src/child_view_render_box.dart
+++ b/public/dart/fuchsia_scenic_flutter/lib/src/child_view_render_box.dart
@@ -186,4 +186,7 @@
 
   @override
   S find<S>(Offset regionOffset) => null;
+
+  @override
+  Iterable<S> findAll<S>(Offset regionOffset) => <S>[];
 }