Add fast path for setting length

src/accessors.h

 // Copyright 2012 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.
 
 #ifndef V8_ACCESSORS_H_
 #define V8_ACCESSORS_H_
 
 #include "src/allocation.h"
 #include "src/globals.h"
 
@@ skipping 80 matching lines @@
   static Handle<AccessorInfo> MakeAccessor(
       Isolate* isolate,
       Handle<Name> name,
       AccessorNameGetterCallback getter,
       AccessorNameSetterCallback setter,
       PropertyAttributes attributes);
 
   static Handle<ExecutableAccessorInfo> CloneAccessor(
       Isolate* isolate,
       Handle<ExecutableAccessorInfo> accessor);
-
-
- private:
-  // Helper functions.
-  static Handle<Object> FlattenNumber(Isolate* isolate, Handle<Object> value);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ACCESSORS_H_