Replace setImmediate with queueMicrotask.

lib/index.js

@@ -1,7 +1,5 @@
 "use strict";
 
-require(`setimmediate`);
-
 // An order represents a pending push or shift.
 const Order = (channel) => {
   let order;
@@ -249,7 +247,7 @@ const Channel = function(length = 0) {
       shift: function() {
         const { order, promise } = Order(this);
         shifts.push(order);
-        setImmediate(processOrders);
+        queueMicrotask(processOrders);
 
         // Don't freeze promise because Bluebird expects it to be mutable.
         return promise;
@@ -321,7 +319,7 @@ const Channel = function(length = 0) {
             processOrders();
 
             // Give remaining orders in flight time to resolve before returning.
-            setImmediate(resolve);
+            queueMicrotask(resolve);
           }
         }),
 
@@ -348,7 +346,7 @@ const Channel = function(length = 0) {
           order.reject(new Error(`Can't push more than one value at a time.`));
         } else {
           pushes.push(order);
-          setImmediate(processOrders);
+          queueMicrotask(processOrders);
         }
 
         // Don't freeze promise because Bluebird expects it to be mutable.