Fix nonspecific type annotation in _getNonPromotionReasons.

Without this change, the local variable `nonPromotionHistory` winds up
getting type `NonPromotionHistory<dynamic>?`, and then the assignment
`Type nonPromotedType = nonPromotionHistory.type;` winds up performing
an implicit dynamic downcast.

Change-Id: I4ece2433ac90623b22aa14cff3e1bfe16c9b491c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389260
Reviewed-by: Kallen Tu <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -5821,7 +5821,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
           Map<Type, NonPromotionReason> result = <Type, NonPromotionReason>{};
           Type currentType = currentPromotionInfo.promotedTypes?.last ??
               operations.variableType(variable);
-          NonPromotionHistory? nonPromotionHistory =
+          NonPromotionHistory<Type>? nonPromotionHistory =
               currentPromotionInfo.nonPromotionHistory;
           while (nonPromotionHistory != null) {
             Type nonPromotedType = nonPromotionHistory.type;