Taylor, got everything. I can build IDE using ant, all good. Tried to run your setup.sh script got error at end:
cp: cannot stat '/home/mrh/flutter/flutterarduino/Flutter' no such file or directory
build fails. Looking at the cloned repository there is no "Flutter" folder. In the setup.sh looks like the offending command is:
sudo -u $user cp -r $SCRIPT_DIR/Flutter $librariesFolder/
$SCRIPT_DIR is where setup.sh is but there is no Flutter dir beneath that. What folder are you trying to copy from? My clone has app, build, core, hardware, libraries.