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

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 bd6682bcbb1d65c3acdcc65a0a3cf5504f15cf0e..8567a02420faa500286742aa057e985c4e98341e 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -652,11 +652,9 @@ class BestPracticesVerifier extends RecursiveAstVisitor<Object> {
     }
     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);