WIP Improve error message thrown in Fiber with multiple copies of React

**what is the change?:**
Adding an 'invariant' with detailed error message for the problem that
occurs when you load two copies of React with the Fiber reconciler.

WIP:
 - Is there any other likely cause for this error besides two copies of
   React?
 - How can we make the message more clear?

Still TODO:
 - Write a unit test
 - Write a documentation page and create the link to the page, similar
   to https://facebook.github.io/react/warnings/refs-must-have-owner.html

It would also be nice to have a page with instructions on how to fix
common cases of accidental double-loading of React, but we can open a
separate issue on that and let the community do it.

**why make this change?:**
This error comes up relatively often and we want to make things clear
when it happens in v16.0+

**test plan:**
WIP

**issue:**
Fixes facebook#9962

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -557,6 +557,12 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   function commitAttachRef(finishedWork: Fiber) {
     const ref = finishedWork.ref;
     if (ref !== null) {
+      invariant(
+        typeof ref === 'function',
+        'commitAttachRef(...): Expected ref to be a function. ' +
+          'You may have multiple copies of React loaded. ' +
+          '(details: https://fb.me/react-expected-ref-to-be-function).',
+      );
       const instance = finishedWork.stateNode;
       switch (finishedWork.tag) {
         case HostComponent: