Ignore _printEvent as an unused element

This is required for #37116

Change-Id: Ia061a423b2455f4e64d5a90f35cc5d7461fa57d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127343
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

Author: srawlins

pkg/front_end/lib/src/fasta/type_inference/type_promotion.dart

@@ -450,6 +450,7 @@ abstract class TypePromoterImpl extends TypePromoter {
 
   /// For internal debugging use, prints the current state followed by the event
   /// name.
+  // ignore: unused_element
   void _printEvent(String name) {
     Iterable<TypePromotionFact> factChain(TypePromotionFact fact) sync* {
       while (fact != null) {