Can the Macbook 12 inch handle XCode/Eclipse for Android/iOS development? I know that Macbook 12 inch was created for battery life and it's lightness instead of performance, but I am really curious about it's performance for development purposes.

Did anyone actually test it on the real device? If so, what do you think? If not, what's your opinion anyways?

While XCode is compiling, will I be able to open safari without seeing "the rainbow"? Slow compilation is not a problem, but I want to know if it hogs the whole OS or not.
The question is quite opinion based. Can it handle development - absolutely. Will it be fast enough - now that's a matter of your personal reception of "fast enough".

The MacBook is quite a capable machine. It will run Development-Tools without any issues. It will perform lower than a MacBook Pro CPU and GPU-wise, so it really depends on your personal needs. Use something like Activity Monitor to keep an eye on your current system load during your daily tasks. If your machine is often at it's limits, you will have to take sacrifices with a MacBook. If your machine idles a lot and is only pushed to the edge rarely, you'll probably be happy with the machine.