Fix bug when @protected is not on a class member (issue 25882)

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 634 matching lines @@
   /**
    * Produces a hint if the given identifier is a protected field or getter
    * accessed outside a subclass.
    */
   void _checkForInvalidProtectedPropertyAccess(SimpleIdentifier identifier) {
     if (identifier.inDeclarationContext()) {
       return;
     }
     Element element = identifier.bestElement;
     if (element is PropertyAccessorElement &&
+        element.enclosingElement is ClassElement &&
         (element.isProtected || element.variable.isProtected)) {
       ClassElement definingClass = element.enclosingElement;
-      if (definingClass == null) {
-        return;
-      }
       ClassDeclaration accessingClass =
           identifier.getAncestor((AstNode node) => node is ClassDeclaration);
 
       if (accessingClass == null) {
         _errorReporter.reportErrorForNode(
             HintCode.INVALID_USE_OF_PROTECTED_MEMBER,
             identifier,
             [identifier.name.toString(), definingClass.name]);
       } else if (!_hasSuperClassOrMixin(
           accessingClass.element, definingClass.type)) {
@@ skipping 12317 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }