The Alert control has been moved to mx.swc, in the frameworks/libs/mx directory. Mx.swc should be on the library path unless your project is configured as "Spark Only" in the Flex Build Path properties.
You were right, I had "Spark Only". Now the Alert works just fine again
as you told i externally add the mx.swc file in my project.
after this i clean my project and my project swc file is deleted .
so how to overcome from this problem.
thanks & regards,
Maybe it was a bug. Get the final build.