Ashley - What version of ams.js is being used by construct? I found a recent build featuring the most recent version of box2d. It seems like a completely automatic build using emscripten. The bindings are also auto generated.
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It's based on https://github.com/kripken/box2d.js/, but funnily enough, it looks like it got an update a few weeks ago! I hadn't realised. I'll look in to what's changed for the next beta cycle.