Add optional import

- Fixes #15

Author: kevinbeaty

optional.js

@@ -0,0 +1,6 @@
+"use strict";
+try {
+  module.exports = require('./register')().Promise || null
+} catch(e) {
+  module.exports = null
+}

test-browser/optional.js

@@ -0,0 +1,13 @@
+'use strict';
+window.Promise = undefined
+var assert = require('assert')
+
+it('Optional test in browser', function(){
+  var Promise = require('../optional')
+  assert.ok(!Promise)
+  assert.throws(function(){
+    var Promise = require('../')
+  }, 'Expecting bare require to throw ');
+  Promise = require('../optional')
+  assert.ok(!Promise)
+})

test/browser.js

@@ -6,7 +6,7 @@ var {spawn} = require('child_process')
 if (+version[1] <= 5) {
   test('skipped browser tests for Node.js <= 5', () => {})
 } else {
-  ;['register', 'local', 'polyfill', 'shortcut'].forEach(filename => {
+  ;['register', 'local', 'polyfill', 'optional', 'shortcut'].forEach(filename => {
     test.serial('Browser test: '+filename, () => zuul(filename))
   })
 }

test/optional.js

@@ -0,0 +1,11 @@
+var test = require('ava')
+var Promise = require('../optional')
+var isPromise = require('is-promise')
+
+test(t => {
+  // optional should always be defined based on current setup.
+  t.is(Promise, global.Promise)
+  t.truthy(isPromise(new Promise(() => {})))
+  t.truthy(Promise.all)
+  t.truthy(global['@@any-promise/REGISTRATION'].implementation)
+})