Apk Build UI 

Regarding  #3351
![screenshot from 2014-09-09 16 54 46 v01](https://cloud.githubusercontent.com/assets/8182792/4210875/08b1dca0-387d-11e4-8463-3119a6d60989.png)

review @ussuri @devoncarew 

This function promises to return a `Future<chrome.ChromeFileEntry>`, but returns a `Future<String>` instead. I can see existing examples of this above: this must have slipped through the reviewing process somehow. @devoncarew WDYT?