[es6] Better support for built-ins subclassing.

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index 28726cba56672788e404fa8a0e72af92cd48ec54..43b74bf8383a2140447b7fc47f6ed1d8a4883021 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -992,7 +992,7 @@ static Object* Runtime_NewObjectHelper(Isolate* isolate,
   // Handle stepping into constructors if step into is active.
   if (debug->StepInActive()) debug->HandleStepIn(function, true);
 
-  if (function->has_initial_map()) {
+  if (function->has_initial_map() && original_function->has_initial_map()) {

[Toon Verwaest, 2015/10/29 12:54:46]

Do we still need this additional special case now it's already handled below?

[Igor Sheludko, 2015/10/29 15:26:22]

Done.

     if (function->initial_map()->instance_type() == JS_FUNCTION_TYPE) {
       // The 'Function' function ignores the receiver object when
       // called using 'new' and creates a new JSFunction object that
@@ -1013,26 +1013,17 @@ static Object* Runtime_NewObjectHelper(Isolate* isolate,
   // available.
   Compiler::Compile(function, CLEAR_EXCEPTION);
 
-  Handle<JSObject> result;
-  if (site.is_null()) {
-    result = isolate->factory()->NewJSObject(function);
-  } else {
-    result = isolate->factory()->NewJSObjectWithMemento(function, site);
-  }
+  JSFunction::EnsureHasInitialMap(function);
+  Handle<Map> initial_map =
+      JSFunction::EnsureDerivedHasInitialMap(original_function, function);
 
-  // Set up the prototoype using original function.
-  // TODO(dslomov): instead of setting the __proto__,
-  // use and cache the correct map.
-  if (*original_function != *function) {
-    if (original_function->has_instance_prototype()) {
-      Handle<Object> prototype =
-          handle(original_function->instance_prototype(), isolate);
-      MAYBE_RETURN(JSObject::SetPrototype(result, prototype, false,
-                                          Object::THROW_ON_ERROR),
-                   isolate->heap()->exception());
-    }
+  if (initial_map->instance_type() == JS_FUNCTION_TYPE) {
+    return isolate->global_proxy();
   }
 
+  Handle<JSObject> result =
+      isolate->factory()->NewJSObjectFromMap(initial_map, NOT_TENURED, site);
+
   isolate->counters()->constructed_objects()->Increment();
   isolate->counters()->constructed_objects_runtime()->Increment();