Fix Function subclassing.

src/runtime/runtime-function.cc

Index: src/runtime/runtime-function.cc
diff --git a/src/runtime/runtime-function.cc b/src/runtime/runtime-function.cc
index b294835916124dd11dd2c6dba11cf32e0b4aa8aa..73e690a7b917314fdfb742a6ac44499406efa805 100644
--- a/src/runtime/runtime-function.cc
+++ b/src/runtime/runtime-function.cc
@@ -77,10 +77,13 @@ RUNTIME_FUNCTION(Runtime_CompleteFunctionConstruction) {
       JSFunction::GetDerivedMap(isolate, constructor, new_target));
 
   Handle<SharedFunctionInfo> shared_info(func->shared(), isolate);
+
+  Handle<Map> function_map =
+      Map::AsLanguageMode(initial_map, shared_info->language_mode());
   Handle<Context> context(func->context(), isolate);
   Handle<JSFunction> result =
       isolate->factory()->NewFunctionFromSharedFunctionInfo(
-          initial_map, shared_info, context, NOT_TENURED);
+          function_map, shared_info, context, NOT_TENURED);
   DCHECK_EQ(func->IsConstructor(), result->IsConstructor());
   return *result;
 }