[Fiber] Calling ReactNative.findNodeHandle() in render() crashes in DEV

[gaearon]

Found this while working on #10517.
See #10517 (comment) for more details.

Now I know this has been fixed in #10448 but that’s not the whole story.

Even if we fixed the Stack-assuming getName() call, we still can't put arbitrary fields on a fiber:

For the reference, this is how we work around the problem in ReactDOM.

[bvaughn]

I'm on it.