Nope. It will recompile everything.
Here is what I would consider: Our ant tasks have an option to provide a source directory. When you do, they recurse, look for any .as class and, if it is a test, test it. You could probably just add to that to test a single class instead of a directory.
Then you would have what you want through ant,
Interesting, this looks like it could be the way forward. How does the task know which SDK to compile with, and what about more advanced compilation options (like
define, which we use occasionally for conditional compilation).
And, on the general subject, would it be possible to start FlashBuilder debugging from the ant script?