Support & test import() for non-bundled code (facebook#1615)

* Test basic import syntax

* Compile import() in test, only support syntax otherwise

fixtures/kitchensink/src/features/syntax/Promises.test.js

@@ -1,12 +1,13 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
-import Promises from './Promises';
 
 describe('promises', () => {
   it('renders without crashing', () => {
     const div = document.createElement('div');
-    return new Promise(resolve => {
-      ReactDOM.render(<Promises onReady={resolve} />, div);
+    return import('./Promises').then(({ default: Promises }) => {
+      return new Promise(resolve => {
+        ReactDOM.render(<Promises onReady={resolve} />, div);
+      });
     });
   });
 });