fix #27040, promote the upper bound of a type parameter

pkg/analyzer/lib/src/generated/resolver.dart

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index 3e1dc6bfb3ff04166ad35619c0e6ca7e38eda4ec..8013c4c08be7a9ef3b632ca9e6788c518eaf70da 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -7584,9 +7584,10 @@ class ResolverVisitor extends ScopedVisitor {
       potentialType ??= DynamicTypeImpl.instance;
 
       // Check if we can promote to potentialType from type.
-      if (typeSystem.canPromoteToType(potentialType, type)) {
+      DartType promoteType = typeSystem.tryPromoteToType(potentialType, type);
+      if (promoteType != null) {
         // Do promote type of variable.
-        _promoteManager.setType(element, potentialType);
+        _promoteManager.setType(element, promoteType);
       }
     }
   }