enable lint avoid_classes_with_only_static_members (#12602)

analysis_options.yaml

@@ -79,7 +79,7 @@ linter:
     - avoid_as
     # - avoid_catches_without_on_clauses # we do this commonly
     # - avoid_catching_errors # we do this commonly
-    # - avoid_classes_with_only_static_members # not yet tested
+    - avoid_classes_with_only_static_members
     - avoid_function_literals_in_foreach_calls
     - avoid_init_to_null
     - avoid_null_checks_in_equality_operators

analysis_options_repo.yaml

@@ -73,7 +73,7 @@ linter:
     - avoid_as
     # - avoid_catches_without_on_clauses # we do this commonly
     # - avoid_catching_errors # we do this commonly
-    # - avoid_classes_with_only_static_members # not yet tested
+    - avoid_classes_with_only_static_members
     - avoid_function_literals_in_foreach_calls
     - avoid_init_to_null
     - avoid_null_checks_in_equality_operators

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

@@ -37,6 +37,8 @@ class _PointerState {
 /// [dart:ui.Window.onPointerDataPacket], and converts them to [PointerEvent]
 /// objects.
 class PointerEventConverter {
+  PointerEventConverter._();
+
   // Map from platform pointer identifiers to PointerEvent pointer identifiers.
   static final Map<int, _PointerState> _pointers = <int, _PointerState>{};