What is FlexUnit .9?
FlexUnit .9 was the first unit testing framework for Flex development. It was originally developed by Alistair Mcleod, who was working for a company named iteration::two at the time.
Writtein in the JUnit 3.x tradition, FlexUnit tests were built by extending an existing core Flex class. The tests reported their results back to a graphical runner that displayed success or failures. As is the case with Fluint, if you are planning to use FlexUnit for your unit testing, use FlexUnit 4. FlexUnit 4 maintains solid backwards compatibility with both FlexUnit .9 and Fluint.