Only use vertxNext() when it is not undefined

Author: Thomas Conner

lib/es6-promise/asap.js

@@ -45,9 +45,13 @@ function useNextTick() {
 
 // vertx
 function useVertxTimer() {
-  return function() {
-    vertxNext(flush);
-  };
+  if (typeof vertxNext !== 'undefined') {
+    return function() {
+      vertxNext(flush);
+    };
+  }
+
+  return useSetTimeout();
 }
 
 function useMutationObserver() {