Avoid MaxListenersExceededWarning in Sequence

Fixes #1345.

lib/sequence.js

@@ -1,5 +1,29 @@
 'use strict';
 
+const beforeExitSubscribers = new Set();
+const beforeExitHandler = () => {
+	for (const subscriber of beforeExitSubscribers) {
+		subscriber();
+	}
+};
+const onBeforeExit = subscriber => {
+	if (beforeExitSubscribers.size === 0) {
+		// Only listen for the event once, no matter how many Sequences are run
+		// concurrently.
+		process.on('beforeExit', beforeExitHandler);
+	}
+
+	beforeExitSubscribers.add(subscriber);
+	return {
+		dispose() {
+			beforeExitSubscribers.delete(subscriber);
+			if (beforeExitSubscribers.size === 0) {
+				process.removeListener('beforeExit', beforeExitHandler);
+			}
+		}
+	};
+};
+
 class Sequence {
 	constructor(runnables, bail) {
 		if (!Array.isArray(runnables)) {
@@ -14,16 +38,15 @@ class Sequence {
 		const iterator = this.runnables[Symbol.iterator]();
 
 		let activeRunnable;
-		const onBeforeExit = () => {
+		const beforeExit = onBeforeExit(() => {
 			if (activeRunnable.finishDueToInactivity) {
 				activeRunnable.finishDueToInactivity();
 			}
-		};
-		process.on('beforeExit', onBeforeExit);
+		});
 
 		let allPassed = true;
 		const finish = () => {
-			process.removeListener('beforeExit', onBeforeExit);
+			beforeExit.dispose();
 			return allPassed;
 		};