Flash 10 uses the same language as Flash 9. Its libraries have lots more features, especially in the graphics system. There's now a low-level graphics API that offers partial 3D, higher performance, and pixel shaders. I was slow to move to Flash 9 in part because the adoption of Flash 9 was slow. It looks like Flash now has auto-updating, and Flash 10 is being installed much more widely. I'm switching all my current projects to Flash 10.
For Flash 10 I'm using the free Flex 3.3 SDK and the Flex
3.3 docs (online or download). The
SDK comes with a command line compiler,
mxmlc, that I run
with mxmlc -target-player 10 on the “main” program, and
that will also compile anything else that is used by the main
class. If you want a tutorial for using
senoular's mxmlc beginner
Flex also comes with a compilation shell,
fcsh, that lets
you keep the compiler in memory to avoid the 2 seconds to start it up
every time you want to recompile. I wrote a wrapper around this so
that whenever I save something in Emacs, it automatically recompiles.
That way, my development cycle is: edit, save, and reload in the
browser. It's quite nice to have a fast cycle.
I haven't played much with the Flash 10 library additions, but the first thing I used was the bitmap line support. I'm using it to draw dashed lines as striped lane dividers. I plan to read about the new library features, but not try them until I find a possible use for them in my projects.
Update: [2013-04] This seems to be the new command line Actionscript compiler, for Flash 11 / Stage3D / AIR.