support async functions (#492)

src/Module.js

@@ -16,7 +16,7 @@ import ModuleScope from './ast/scopes/ModuleScope.js';
 function tryParse ( code, comments, acornOptions, id ) {
 	try {
 		return parse( code, assign({
-			ecmaVersion: 7,
+			ecmaVersion: 8,
 			sourceType: 'module',
 			onComment: ( block, text, start, end ) => comments.push({ block, text, start, end }),
 			preserveParens: false

test/form/async-function-unused/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'treeshakes async functions (#492)'
+};

test/form/async-function-unused/_expected/amd.js

@@ -0,0 +1,9 @@
+define(function () { 'use strict';
+
+	async function foo () {
+		return 'foo';
+	}
+
+	foo().then( value => console.log( value ) );
+
+});

test/form/async-function-unused/_expected/cjs.js

@@ -0,0 +1,7 @@
+'use strict';
+
+async function foo () {
+	return 'foo';
+}
+
+foo().then( value => console.log( value ) );

test/form/async-function-unused/_expected/es.js

@@ -0,0 +1,5 @@
+async function foo () {
+	return 'foo';
+}
+
+foo().then( value => console.log( value ) );

test/form/async-function-unused/_expected/iife.js

@@ -0,0 +1,10 @@
+(function () {
+	'use strict';
+
+	async function foo () {
+		return 'foo';
+	}
+
+	foo().then( value => console.log( value ) );
+
+}());

test/form/async-function-unused/_expected/umd.js

@@ -0,0 +1,13 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, (function () { 'use strict';
+
+	async function foo () {
+		return 'foo';
+	}
+
+	foo().then( value => console.log( value ) );
+
+})));

test/form/async-function-unused/main.js

@@ -0,0 +1,3 @@
+import { foo } from './utils.js';
+
+foo().then( value => console.log( value ) );

test/form/async-function-unused/utils.js

@@ -0,0 +1,7 @@
+export async function foo () {
+	return 'foo';
+}
+
+export async function bar () {
+	return 'bar';
+}