RABCDAsm (https://github.com/CyberShadow/RABCDAsm) is a collection of utilities including an ActionScript 3 assembler/disassembler, and a few tools to manipulate SWF files. WinRABCDAsm is a GUI front-end for its core functions. For syntax highlighting see http://ohadsc.wordpress.com/2012/05/26/adding-syntax-highlighting-for-new-languages-to-eclipse-with-the-colorer-library/ and https://github.com/CyberShadow/RABCDAsm/blob/master/asasm.hrc
Flasm is an assembler/disassembler for Flash actionscript bytecode.
Using flasm, script authors may learn how actionscript compiler and Flash Player virtual machine work, and tweak the compiled bytecode in SWF files for performance.