Evergreen diskstats fails on OSX

The run_diskstats in etc/evergreen.yml fails on OSX:

[2016/08/24 02:36:54.182] if [ "Windows_NT" = "$OS" ]; then
[2016/08/24 02:36:54.183] typeperf -qx PhysicalDisk | grep Disk | grep -v _Total > disk_counters.txt
[2016/08/24 02:36:54.183] typeperf -cf disk_counters.txt -si 5 -o mongo-diskstats
[2016/08/24 02:36:54.183] # See if iostat is available.
[2016/08/24 02:36:54.183] elif which iostat 2>&1 /dev/null; then
[2016/08/24 02:36:54.183] iostat -d 5 > mongo-diskstats
[2016/08/24 02:36:54.183] # Fall back to vmstat if iostat is not available.
[2016/08/24 02:36:54.183] else
[2016/08/24 02:36:54.183] vmstat -d 5 > mongo-diskstats
[2016/08/24 02:36:54.183] fi
[2016/08/24 02:36:54.184] /usr/sbin/iostat
[2016/08/24 02:36:54.185] sh: line 13: vmstat: command not found