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

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 6869 matching lines @@
         return;
       }
       // prepare current variable type
       DartType type = _promoteManager.getType(element) ??
           expression.staticType ??
           DynamicTypeImpl.instance;
 
       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);
       }
     }
   }
 
   /**
    * Promotes type information using given condition.
    */
   void _promoteTypes(Expression condition) {
     if (condition is BinaryExpression) {
       if (condition.operator.type == TokenType.AMPERSAND_AMPERSAND) {
@@ skipping 3817 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }