Upgrade Google closure compiler

[twwwt]

Would it be possible to make a new release of the plugin which comes with a more recent version of Google's closure compiler, ideally the most recent version v20180101. Lots of ES6 improvements since the version from that is currently referenced.

[twwwt]

Let me bump this issue. We are still using the plugin and would highly appreciate a minor new release of the plugin where versions would be upgraded. In case it helps I can create a pull request.