skpm build `import/export` error
example codes

```js
// common/util.js
const app = NSApplication.sharedApplication();

export function noop() {

}

export function alert(title, message) {    
    app.displayDialog_withTitle(message, title);
}
```

then import the local module

```js
import util from './common/util';

export default function(context) {
    util.alert('This is an alert box', 'Alert Box title');
}
```

the compiled codes be 

```js
// .... ignore other codes
exports['default'] = function (context) {
    _util2['default'].alert('This is an alert box', 'Alert Box title');
};

var _util = __webpack_require__(1);

var _util2 = _interopRequireDefault(_util);

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { 'default': obj }; }

/***/ }),
/* 1 */
/***/ (function(module, exports) {

Object.defineProperty(exports, "__esModule", {
    value: true
});
exports.noop = noop;
exports.alert = alert;
var app = NSApplication.sharedApplication();

function noop() {}

function alert(title, message) {
    app.displayDialog_withTitle(message, title);
}

/***/ })
/******/ ]);
  if (key === 'default' && typeof exports === 'function') {
    exports(context);
  } else {
    exports[key](context);
  }
}
that['onRun'] = __skpm_run.bind(this, 'default')
```

**the problem is:**

`_util2['default'].alert('This is an alert box', 'Alert Box title');` should compiled to `_util2.alert('This is an alert box', 'Alert Box title');`

`_util2['default']` is undefined!