Add regression test for RegExp LegacyFeaturesEnabled flag.

Author: woess

graal-js/src/com.oracle.truffle.js.test/js/regexp_legacy_features.js

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2023, 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * Licensed under the Universal Permissive License v 1.0 as shown at http://oss.oracle.com/licenses/upl.
+ */
+
+/**
+ * Test regexp-legacy-features behavior.
+ */
+
+load("assert.js");
+
+class MyRegExp extends RegExp {}
+
+// TypeError: RegExp.prototype.compile cannot be used on subclasses of RegExp.
+assertThrows(() => new MyRegExp(/foo/).compile('pattern', ''), TypeError);
+assertThrows(() => new MyRegExp('foo').compile('pattern', ''), TypeError);