When you compile your Flex application, it removes all the import statements from your script.
This may also happen if you are using some class/its property in the MXML file, while the import statement for that class is written in your .as script file. In this case when u compile, the import statement get removed. Let me know if this helped you.
The problem comes after 3.4 sdk update. for ex if you type import flash you will get only flash.erros and others will go off. also for any component in mxml file wont get auto complete for click and some other events. i am able to solve only if i go back to flex sdk 3.3 or older.
The issue came when the flex builder installed with sdk 3 and later upgraded the sdk to higher version. To solve this, please uninstall the flex builder and install flex builder with 3.2 sdk (it will automatically install 2.0.1 hotfix 3) (download from http://www.adobe.com/support/documentation/en/flex/3/releasenotes_flex3_fb.html). Then upgrade the sdk to 3.5
It works now.