Avoid some unnecessary fast-properties map creations.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 258390c4d5cb2b62ff079e31d90de3e5595b6fcb..2b17ebb8b12d816c4401ecffdb18f861fc088a15 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9416,13 +9416,17 @@ void JSObject::OptimizeAsPrototype(Handle<JSObject> object,
     // First normalize to ensure all JSFunctions are CONSTANT.
     JSObject::NormalizeProperties(object, KEEP_INOBJECT_PROPERTIES, 0);
   }
+  bool has_just_copied_map = false;
   if (!object->HasFastProperties()) {
     JSObject::MigrateSlowToFast(object, 0);
+    has_just_copied_map = true;
   }
   if (mode == FAST_PROTOTYPE && object->HasFastProperties() &&
       !object->map()->is_prototype_map()) {
-    Handle<Map> new_map = Map::Copy(handle(object->map()));
-    JSObject::MigrateToMap(object, new_map);
+    if (!has_just_copied_map) {
+      Handle<Map> new_map = Map::Copy(handle(object->map()));
+      JSObject::MigrateToMap(object, new_map);
+    }
     object->map()->set_is_prototype_map(true);
   }
 }