var html = HtmlService.createHtmlOutputFromFile('foo');
We couldn’t resist using the new HTML service ourselves, so we’ve published an alternate version of the Google Translate add-on on GitHub that uses the iframe sandbox mode as well as Polymer and material design concepts.
With the launch of the IFRAME sandbox, it is now time to bid goodbye to UiApp. The UI service was a very useful way to serve user interfaces in Docs and Sheets at a time when sandboxing technologies were still evolving. Effective today, UI service is deprecated. It will be removed from documentation and autocomplete in the script editor on June 30, 2015. If you have built any UIs using UiApp, they will continue to work.