Doesn't really matter if the client is open source does it?
The compiler is the interesting part here, crowd funding a third party proprietary service is a bit odd.
I'm not sure anyone would really trust code being built on a third party website without transparency. Nothing is to stop you from injecting shady code, etc.
Anyway, my two cents: the compiler is interesting but the service is not. Best of luck.
I'm pretty sure the Arduino example works, as it looks quite similar to Arduino code. Now what would be interesting is if the hello world example would compile for Arduino (eg. Console.log being handled by a serial.write)