chore: make the $parser non-enumerable

lib/rule-tester/rule-tester.js

@@ -239,6 +239,21 @@ function defineStartEndAsError(objName, node) {
     });
 }
 
+/**
+ * define a property $parser to be original parser
+ * use `defineProperty` to make it non-enumerable
+ * @param {Parser} wrappedParser the wrapped parser
+ * @param {Parser} originalParser the original parser
+ * @returns {void}
+ */
+function defineOriginalParser(wrappedParser, originalParser) {
+    Object.defineProperty(wrappedParser, "$parser", {
+        value: originalParser,
+        writable: true,
+        enumerable: false
+    });
+}
+
 /**
  * Define `start`/`end` properties of all nodes of the given AST as throwing error.
  * @param {ASTNode} ast The root node to errorize `start`/`end` properties.
@@ -258,8 +273,10 @@ function defineStartEndAsErrorInTree(ast, visitorKeys) {
  * @returns {Parser} Wrapped parser object.
  */
 function wrapParser(parser) {
+    let wrappedParser;
+
     if (typeof parser.parseForESLint === "function") {
-        return {
+        wrappedParser = {
             $parser: parser,
             parseForESLint(...args) {
                 const ret = parser.parseForESLint(...args);
@@ -268,16 +285,22 @@ function wrapParser(parser) {
                 return ret;
             }
         };
+    } else {
+        wrappedParser = {
+            $parser: parser,
+            parse(...args) {
+                const ast = parser.parse(...args);
+
+                defineStartEndAsErrorInTree(ast);
+                return ast;
+            }
+        };
     }
-    return {
-        $parser: parser,
-        parse(...args) {
-            const ast = parser.parse(...args);
 
-            defineStartEndAsErrorInTree(ast);
-            return ast;
-        }
-    };
+    defineOriginalParser(wrappedParser, parser);
+
+    return wrappedParser;
+
 }
 
 //------------------------------------------------------------------------------