fix #25486, promote from dynamic in strong mode

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/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 9705 matching lines @@
    * @param potentialType the potential type of the elements
    */
   void _promote(Expression expression, DartType potentialType) {
     VariableElement element = getPromotionStaticElement(expression);
     if (element != null) {
       // may be mutated somewhere in closure
       if (element.isPotentiallyMutatedInClosure) {
         return;
       }
       // prepare current variable type
-      DartType type = _promoteManager.getType(element);
-      if (type == null) {
-        type = expression.staticType;
+      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)) {
+        // Do promote type of variable.
+        _promoteManager.setType(element, potentialType);
       }
-      // Declared type should not be "dynamic".
-      if (type == null || type.isDynamic) {
-        return;
-      }
-      // Promoted type should not be "dynamic".
-      if (potentialType == null || potentialType.isDynamic) {
-        return;
-      }
-      // Promoted type should be more specific than declared.
-      if (!potentialType.isMoreSpecificThan(type)) {
-        return;
-      }
-      // Do promote type of variable.
-      _promoteManager.setType(element, potentialType);
     }
   }
 
   /**
    * Promotes type information using given condition.
    */
   void _promoteTypes(Expression condition) {
     if (condition is BinaryExpression) {
       BinaryExpression binary = condition;
       if (binary.operator.type == TokenType.AMPERSAND_AMPERSAND) {
@@ skipping 3851 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }