With flex2ant you have the ability to add the Adobe Flex 2 SDK command line compiler (mxmlc) to your Eclipse installation. By installing flex2ant you add a specific Ant task to the Ant installation that is part of the Eclipse IDE, therefore making this task visible to the Eclipse environment.

And here is a tutorials about how to add the Flex2Ant plugin into Flex Builder.The plugin will automatically create an Ant  build file from your Flex Builder project,it’s so cool ! Just right-click on your project’s name and select the option Generate Ant build file.