Cannot set property onunhandledrejection of [object Object] which has only a getter

This may have birthed from [Issue#35](https://github.com/GoogleCloudPlatform/stackdriver-errors-js/issues/35).

I'm seeing logs with the following error:
```
TypeError: Cannot set property onunhandledrejection of [object Object] which has only a getter
at onunhandledrejection (../node_modules/stackdriver-errors-js/stackdriver-errors.js:91:0)
at window.addEventListener (index.js:133:15)
```

This is happening on my starting the ErrorReporter. index.js:133 is line 3:
```
window.addEventListener('DOMContentLoaded', function () {
var errorHandler = new StackdriverErrorReporter();
errorHandler.start({
targetUrl: '',
});
})
```

**User Agent**: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36