In particular, when dealing with Flash, feature-completeness is going to be much more important than perfect performance. Since the Flash VM on Linux and OS X is so slow and software-renders everything, it really won't take much to improve on it. The advantages of a dynamic language are probably going to far outweigh C++'s speed, and Javascript's similarity to Actionscript is icing.

Plus, Javascript can take advantage of whatever hardware-accelerated graphics library the browser vendor chooses.