Keep function.prototype fast.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 42b3913e3a2528c6b536e0b27743b747e4db7aa2..32746b0c7a0d9e2100e2a70e47beb80b5d98a992 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2115,6 +2115,7 @@ bool JSObject::HasFastProperties() {
 
 bool Map::TooManyFastProperties(StoreFromKeyed store_mode) {
   if (unused_property_fields() != 0) return false;
+  if (is_prototype_map()) return false;
   int minimum = store_mode == CERTAINLY_NOT_STORE_FROM_KEYED ? 128 : 12;
   int limit = Max(minimum, inobject_properties());
   int external = NumberOfFields() - inobject_properties();
@@ -4445,6 +4446,15 @@ bool Map::is_extensible() {
 }
 
 
+void Map::mark_prototype_map() {
+  set_bit_field2(IsPrototypeMapBits::update(bit_field2(), true));
+}
+
+bool Map::is_prototype_map() {
+  return IsPrototypeMapBits::decode(bit_field2());
+}
+
+
 void Map::set_is_shared(bool value) {
   set_bit_field3(IsShared::update(bit_field3(), value));
 }