I think there must be another possibility then installing the Java Development Tools
but only Ant. Until then thanks for this information.
FB2 without Ant makes not so much sense to me. I prefer to configure the compile statement and other jobs with Ant. The advantage is that this important information can also be stored in a version control system like Subversion or CVS. Otherwise I guess e.g. additional compile arguments are only saved in the Flex Builder itself.
You might argue why I use FB2 and not the plugin for Eclipse. This is a problem when you also use Eclipse for AS2 development and the .as extention in my case is registered to FDT.