Add fast path for setting length

src/accessors.h

Index: src/accessors.h
diff --git a/src/accessors.h b/src/accessors.h
index d37b6b770cafa5db88b3ed2bf3ef0005e90590be..227af745b70a49328fbf9005eb5ed123c873b330 100644
--- a/src/accessors.h
+++ b/src/accessors.h
@@ -98,11 +98,6 @@ class Accessors : public AllStatic {
   static Handle<ExecutableAccessorInfo> CloneAccessor(
       Isolate* isolate,
       Handle<ExecutableAccessorInfo> accessor);
-
-
- private:
-  // Helper functions.
-  static Handle<Object> FlattenNumber(Isolate* isolate, Handle<Object> value);
 };
 
 } }  // namespace v8::internal