I found it incredibly difficult to flash the system images using any publicly available fastboot. It seems all of the current releases of fastboot don't very well support the "slots" for system, boot, vendor, etc (used for background system updates).I had to compile fastboot from source for AOSP master to get this working. If anyone else wants to flash system images, you can grab my compile from here:I also included adb from master, but I had no problems with the regular one. If you just want fastboot, you also need the libwinpthread DLL (this is from AOSP).Feel free to add this to the index. Hope this helps someone besides just me.