This is Adobe's Open Source Flex SDK project.
Flex is a highly productive, open source framework for building and maintaining expressive web applications that deploy consistently on all major browsers, desktops and operating systems.
This project lives in Adobe's open-source Subversion repository at
http://opensource.adobe.com/svn/opensource/flex/sdk.
The website for this project is at
http://opensource.adobe.com/wiki/display/flexsdk/flex+sdk.