Fix crash during server-render in non-Node environments.

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

Author: Dustin Masters

packages/react-scheduler/src/ReactScheduler.js

@@ -60,8 +60,8 @@ if (__DEV__) {
 // this module is initially evaluated.
 // We want to be using a consistent implementation.
 const localDate = Date;
-const localSetTimeout = setTimeout;
-const localClearTimeout = clearTimeout;
+const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;
+const localClearTimeout = typeof clearTimeout === 'function' ? clearTimeout : null;
 
 const hasNativePerformanceNow =
   typeof performance === 'object' && typeof performance.now === 'function';