Also mark as prototype when passing in while creating a function.

src/factory.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/factory.h"
 
 #include "src/allocation-site-scopes.h"
 #include "src/conversions.h"
 #include "src/isolate-inl.h"
 #include "src/macro-assembler.h"
@@ skipping 1250 matching lines @@
 
 
 Handle<JSFunction> Factory::NewFunction(Handle<String> name,
                                         Handle<Code> code,
                                         Handle<Object> prototype,
                                         bool read_only_prototype) {
   Handle<Map> map = read_only_prototype
       ? isolate()->sloppy_function_with_readonly_prototype_map()
       : isolate()->sloppy_function_map();
   Handle<JSFunction> result = NewFunction(map, name, code);
+  if (!prototype->IsTheHole()) {
+    Handle<JSObject> js_proto = Handle<JSObject>::cast(prototype);
+    Handle<Map> new_map = Map::CopyAsPrototypeMap(handle(js_proto->map()));
+    JSObject::MigrateToMap(js_proto, new_map);
+  }
   result->set_prototype_or_initial_map(*prototype);
   return result;
 }
 
 
 Handle<JSFunction> Factory::NewFunction(Handle<String> name,
                                         Handle<Code> code,
                                         Handle<Object> prototype,
                                         InstanceType type,
                                         int instance_size,
@@ skipping 1084 matching lines @@
   return Handle<Object>::null();
 }
 
 
 Handle<Object> Factory::ToBoolean(bool value) {
   return value ? true_value() : false_value();
 }
 
 
 } }  // namespace v8::internal