I don't use NME, so I'm not sure how this fits into the NME build tools, but with simple haxe you would use a build server, for example:
haxe --wait 4000
to start the server, and then when creating add
--connect 4000
While the build server is up and running, the compiler caches the results of compiling the modules, so things like the back of the head will only be compiled for the first time, and until the code is changed, only the compiler only generates .swf
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