[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 423d60e5e20c962fd5ab88303a0a2ef91e19df98..6cda851a9685afbe602c389711f14470984a37f4 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -1033,8 +1033,8 @@ 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->initial_map()->instance_type() == JS_FUNCTION_TYPE) {
+  if (original_function->has_initial_map()) {
+    if (original_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
       // is returned.  The receiver object is only used for error
@@ -1054,24 +1054,12 @@ 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);
-      RETURN_FAILURE_ON_EXCEPTION(
-          isolate, JSObject::SetPrototype(result, prototype, false));
-    }
-  }
+  Handle<JSObject> result =
+      isolate->factory()->NewJSObjectFromMap(initial_map, NOT_TENURED, site);
 
   isolate->counters()->constructed_objects()->Increment();
   isolate->counters()->constructed_objects_runtime()->Increment();