Support mocking native async methods (jestjs#3209)

* Support mocking native AsyncFunction

* Add native-async-mock example test

* Use strict

* Return a promise from mock

* Only test async

* Revert returning promise

examples/async/native-async-mock/.babelrc

@@ -0,0 +1,3 @@
+{
+  "presets": ["es2015"]
+}

examples/async/native-async-mock/__tests__/native-async-mock-test.js

@@ -0,0 +1,11 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+'use strict';
+
+jest.mock('../native');
+
+const native = require('../native');
+
+test('mock works with native async', () => {
+  expect(native.asyncMethod).toBeDefined();
+});

examples/async/native-async-mock/native.js

@@ -0,0 +1,14 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+'use strict';
+
+function awaitable() {
+  return Promise.resolve();
+}
+
+module.exports.syncMethod = () => 42;
+
+module.exports.asyncMethod = async () => {
+  await awaitable();
+  return 42;
+};

packages/jest-mock/src/index.js

@@ -95,7 +95,7 @@ function isA(typeName: string, value: any): boolean {
 }
 
 function getType(ref?: any): string | null {
-  if (isA('Function', ref)) {
+  if (isA('Function', ref) || isA('AsyncFunction', ref)) {
     return 'function';
   } else if (Array.isArray(ref)) {
     return 'array';
@@ -127,7 +127,7 @@ function isReadonlyProp(object: any, prop: string): boolean {
     prop === 'callee' ||
     prop === 'name' ||
     prop === 'length') &&
-    isA('Function', object)) ||
+    (isA('Function', object) || isA('AsyncFunction', object))) ||
     ((prop === 'source' ||
       prop === 'global' ||
       prop === 'ignoreCase' ||