I was going to mention that trace() is a shorthand for Log.trace(), but you might still get an error using onComplete(Log.trace, ["Hello, world"]).
The problem is with the “magic” posInfos parameter. If you call Log.trace using reflection, Haxe won’t automagically set posInfos, which will lead to a null pointer error on some platforms.
tl;dr: Ignore this post and stick with tienery’s suggestion.
Yeah, that didn’t work in the Flash target… it’s a bit annoying when using Greensock I would pass a call to the superclass when onComplete, but you can’t do that in Actuate either.