dillon1000/react

Fix crash during server render in react 16.4.1. (#13088)

* Fix crash during server render.

setTimeout and clearTimeout may not be available in some server-render environments (such as ChakraCore in React.NET), and loading ReactScheduler.js will cause a crash unless the existence of the variables are checked via a typeof comparison.

https://github.com/reactjs/React.NET/issues/555

The crash did not occur in 16.4.0, and the change appears to have been introduced here: https://github.com/facebook/react/pull/12931/files#diff-bbebc3357e1fb99ab13ad796e04b69a6L47

I tested this by using yarn link and running it with a local copy of React.NET. I am unsure the best way to unit test this change, since assigning null to `setTimeout` causes an immediate crash within the Node REPL.

* Fix flow errors and log warning if setTimeout / clearTimeout are
not defined / not a function.

* Use invariant to assert setTimeout / clearTimeout are functions

* Remove use of invariant

* Explain