React 16.3: Rewrite Error Handling

In React 16.3, which was just released last week, we rewrote like all of error handling in preparation for this feature. One thing that I've come to rely on more, although I didn't in the error handling case, is fuzz testing. We're very careful to write tests in such a way that they make sense regardless of the implementation. And depending on your CPU, 100 milliseconds could be something totally different on one computer from another.