I didn't write this, but wanted to make sure the information didn't disappear. Currently the Haxe community doesn't use the google group anymore and this information was written down on that group.
I have tested it on OsX, and at first I tried brew
but that failed (have no clue why)
And after that I just followed the instructions on getting started on webassembly.
git clone https://github.com/juj/emsdk.git
cd emsdk
./emsdk install latest
./emsdk activate latest
and wait... a ... while ...
source ./emsdk_env.sh
and then it worked.
Probably you will need to install hxcpp
haxelib install hxcpp
The rest is written by Russ (that's all I know about the writer)