Don't use global setImmediate

It can be modified by test code.

Author: novemberborn

lib/ipc-flow-control.js

@@ -7,6 +7,8 @@
 // minimally supported versions) we need flow control based on `send()`'s return
 // value.
 
+const nowAndTimers = require('./now-and-timers');
+
 function controlFlow(channel) {
 	let sending = false;
 
@@ -32,7 +34,7 @@ function controlFlow(channel) {
 		buffer.push(message);
 		if (!sending) {
 			sending = true;
-			setImmediate(deliverNext);
+			nowAndTimers.setImmediate(deliverNext);
 		}
 	};
 }