Question: I'm looking to use React Native for app development. Since it is free and an open source, cross-platform environment. The question is: what IDE can I use to write and build and do everything in it?
Appreciate the help.

Answer: 
Since I use their IDEs for other languages too I use PhpStore/WebStorm from JetBrains. It's a very good set of tools. As an added bonus Android Studio is using the JetBrains IDE as a basis as well. So you'll feel right at home there. On the downside you have to pay and they just this month switch over to a subscription model. But as a professional developer I'm willing to pay for tools that make me more productive.
sublime with [Babel syntax highlighting](https://github.com/babel/babel-sublime). :)