Add temporary default case to support new PointerSignalKind (flutter#…

…120731)

* add default case to prep for engine pr

* combine unknown w default

* fixed todo comment not inline with flutter standards

packages/flutter/lib/src/gestures/converter.dart

@@ -278,6 +278,8 @@ class PointerEventConverter {
                 scale: datum.scale,
               );
             case ui.PointerSignalKind.unknown:
+            default: // ignore: no_default_cases, to allow adding a new [PointerSignalKind] - PointerStylusAuxiliaryAction
+            // TODO(louisehsu): remove after landing engine PR https://github.com/flutter/engine/pull/39637
               // This branch should already have 'unknown' filtered out, but
               // we don't want to return anything or miss if someone adds a new
               // enumeration to PointerSignalKind.