[WebIDL] Test subclassing of legacy factory functions

Author: ExE-Boss

WebIDL/ecmascript-binding/legacy-factor-function-subclass.window.js

@@ -0,0 +1,13 @@
+"use strict";
+
+test(() => {
+  class CustomImage extends Image {}
+  var instance = new CustomImage();
+
+  assert_equals(
+    Object.getPrototypeOf(instance), CustomImage.prototype,
+    "Object.getPrototypeOf(instance) === CustomImage.prototype");
+
+  assert_true(instance instanceof CustomImage, "instance instanceof CustomImage");
+  assert_true(instance instanceof HTMLImageElement, "instance instanceof HTMLImageElement");
+}, "[LegacyFactoryFunction] can be subclassed and correctly handles NewTarget");