LibWeb: Implement WebIDL constructor flow for derived classes

Following the rules in the algorithm from
https://webidl.spec.whatwg.org/#js-platform-objects, "To Internally
create a new object implementing the interface interface", this change
incorporates the steps to load a prototype from new.target, and write
it to the created instance returned from constructor_impl(). This
mirrors the code for generate_html_constructor(), which incorporates
additional steps needed by Custom Elements.

Bug #334

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -2513,6 +2513,26 @@ JS::ThrowCompletionOr<JS::NonnullGCPtr<JS::Object>> @constructor_class@::constru
     if (is_html_constructor) {
         generate_html_constructor(generator, constructor, interface);
     } else {
+        generator.append(R"~~~(
+    // To internally create a new object implementing the interface @name@:
+
+    // 3.2. Let prototype be ? Get(newTarget, "prototype").
+    auto prototype = TRY(new_target.get(vm.names.prototype));
+
+    // 3.3. If Type(prototype) is not Object, then:
+    if (!prototype.is_object()) {    
+        // 1. Let targetRealm be ? GetFunctionRealm(newTarget).
+        auto* target_realm = TRY(JS::get_function_realm(vm, new_target));
+
+        // 2. Set prototype to the interface prototype object for interface in targetRealm.
+        VERIFY(target_realm);
+        prototype = &Bindings::ensure_web_prototype<@prototype_class@>(*target_realm, "@name@"_fly_string);
+    }
+
+    // 4. Let instance be MakeBasicObject( « [[Prototype]], [[Extensible]], [[Realm]], [[PrimaryInterface]] »).
+    // 5. Set instance.[[Realm]] to realm.
+    // 6. Set instance.[[PrimaryInterface]] to interface.
+)~~~");
         if (!constructor.parameters.is_empty()) {
             generate_argument_count_check(generator, constructor.name, constructor.shortest_length());
 
@@ -2530,6 +2550,34 @@ JS::ThrowCompletionOr<JS::NonnullGCPtr<JS::Object>> @constructor_class@::constru
         }
 
         constructor_generator.append(R"~~~(
+    // 7. Set instance.[[Prototype]] to prototype.
+    VERIFY(prototype.is_object());
+    impl->set_prototype(&prototype.as_object());
+
+    // FIXME: Steps 8...11. of the "internally create a new object implementing the interface @name@" algorithm
+    // (https://webidl.spec.whatwg.org/#js-platform-objects) are currently not handled, or are handled within @fully_qualified_name@::construct_impl().
+    //  8. Let interfaces be the inclusive inherited interfaces of interface.
+    //  9. For every interface ancestor interface in interfaces:
+    //    9.1. Let unforgeables be the value of the [[Unforgeables]] slot of the interface object of ancestor interface in realm.
+    //    9.2. Let keys be ! unforgeables.[[OwnPropertyKeys]]().
+    //    9.3. For each element key of keys:
+    //      9.3.1. Let descriptor be ! unforgeables.[[GetOwnProperty]](key).
+    //      9.3.2. Perform ! DefinePropertyOrThrow(instance, key, descriptor).
+    //  10. If interface is declared with the [Global] extended attribute, then:
+    //    10.1. Define the regular operations of interface on instance, given realm.
+    //    10.2. Define the regular attributes of interface on instance, given realm.
+    //    10.3. Define the iteration methods of interface on instance given realm.
+    //    10.4. Define the asynchronous iteration methods of interface on instance given realm.
+    //    10.5. Define the global property references on instance, given realm.
+    //    10.6. Set instance.[[SetPrototypeOf]] as defined in § 3.8.1 [[SetPrototypeOf]].
+    //  11. Otherwise, if interfaces contains an interface which supports indexed properties, named properties, or both:
+    //    11.1. Set instance.[[GetOwnProperty]] as defined in § 3.9.1 [[GetOwnProperty]].
+    //    11.2. Set instance.[[Set]] as defined in § 3.9.2 [[Set]].
+    //    11.3. Set instance.[[DefineOwnProperty]] as defined in § 3.9.3 [[DefineOwnProperty]].
+    //    11.4. Set instance.[[Delete]] as defined in § 3.9.4 [[Delete]].
+    //    11.5. Set instance.[[PreventExtensions]] as defined in § 3.9.5 [[PreventExtensions]].
+    //    11.6. Set instance.[[OwnPropertyKeys]] as defined in § 3.9.6 [[OwnPropertyKeys]].
+
     return *impl;
 }
 )~~~");

Tests/LibWeb/Text/expected/DOM/derived-interface-prototype.txt

@@ -0,0 +1 @@
+derived class method present and invoked!

Tests/LibWeb/Text/input/DOM/derived-interface-prototype.html

@@ -0,0 +1,19 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        class MyEventTarget extends EventTarget {
+            derivedClassMethod() {
+                println("derived class method present and invoked!");
+            }
+        }
+        let instance = new MyEventTarget();
+        if (instance.__proto__ !== MyEventTarget.prototype) {
+            println("Derived class MyEventTarget has incorrect prototype.");
+        }
+        try {
+            instance.derivedClassMethod();
+        } catch (e) {
+            println(e);
+        }
+    });
+</script>