flex 3.4 compile error when using FileReference object

When I using flex FileReference, in flex 3.4, I met a problem. The follows is my code:

public function save_click():void {

             var systemFileReference:FileReference = new FileReference();


            systemFileReference.save("test开发者_运维知识库","testfile.txt");

        }

But it compile fails" invoke undefined method save in FileReference), I do not know why in Flex3.4 the FileReference does not support save() method?

You need to target Flash Player 10.