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.

reactjs/React.NET#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

packages/react-scheduler/src/ReactScheduler.js

@@ -60,8 +60,15 @@ if (__DEV__) {
 // this module is initially evaluated.
 // We want to be using a consistent implementation.
 const localDate = Date;
-const localSetTimeout = setTimeout;
-const localClearTimeout = clearTimeout;
+
+// This initialization code may run even on server environments
+// if a component just imports ReactDOM (e.g. for findDOMNode).
+// Some environments might not have setTimeout or clearTimeout.
+// https://github.com/facebook/react/pull/13088
+const localSetTimeout =
+  typeof setTimeout === 'function' ? setTimeout : (undefined: any);
+const localClearTimeout =
+  typeof clearTimeout === 'function' ? clearTimeout : (undefined: any);
 
 const hasNativePerformanceNow =
   typeof performance === 'object' && typeof performance.now === 'function';