How Google Inbox does it: Java and GWT
There was a presentation at GWTCreate where Ray Cromwell laid out some of the magic on how they used GWT to handle this. The slides for the talk are here[0]. One of the key pieces here is that coding in Java gets you the web (with GWT), Android, and iOS using j2objc[1]. They posted a demo app showing this off[2].
The videos aren't up for the talks yet, but the Keynote from GWTCreate is posted (Ray gave that talk as well)[3].
[1] https://github.com/google/j2objc