According to the reference Flex has an InteractiveObject class: http://livedocs.adobe.com/flex/3/langref/flash/display/InteractiveObject.html
Then why doesn't the SDK contain a file called InteractiveObject.as ?
It contains that class but you don't have access to the source because but it's in playerglobal.swc (sdkpath\frameworks\libs\player\10\playerglobal.swc).
So it's not open source then?
I'm afraid it isn't open source.