release swf error

I test in the unity,the project is running well,but when i want to release to swf file,the unity tell me an error and can't release well-off; the error is like:

C:\Users\Tao\Documents\PowerPlant\Temp\StagingArea\Data\ConvertedDotNetCode\global\ShowSomething.as(65): col: 39 Error: Access of undefined property File.

C:\Users\Tao\Documents\PowerPlant\Temp\StagingArea\Data\ConvertedDotNetCode\global\ShowSomething.as(5): col: 18 Error: Definition System.IO:File could not be found.

Error building Player: Exception: Compiling SWF Failed: Loading configuration file C:\Program Files\Unity\Editor\Data\PlaybackEngines\flashsupport\BuildTools\flex\frameworks\flex-config.xml

C:\Users\Tao\Documents\PowerPlant\Temp\StagingArea\Data\ConvertedDotNetCode\global\ShowSomething.as(65): col: 39 Error: Access of undefined property File.

var $aXmlString: CLIObjectArray = File.File_ReadAllLines_String(Application.dataPath + "/menuConfig.xml"); ^

C:\Users\Tao\Documents\PowerPlant\Temp\StagingArea\Data\ConvertedDotNetCode\global\ShowSomething.as(5): col: 18 Error: Definition System.IO:File could not be found.

import System.IO.File;

I thinks a long time,but i can't find the bug.If anybody could give a hand,i will be very thanks.