[pavlov]

According to an HN comment by user cromwellian, it's widely used inside Google including the new Inbox mail app:

https://news.ycombinator.com/item?id=8554339

Other references mentioned by the poster are "Apple (iAds Workspace), Amazon (AWS Console), Nike".

[Nitramp]

Though it's worth noting that Inbox and similar properties do not use it for UI code, but only (exclusively) for business logic below the UI layer. UI code is written in pure JavaScript/Closure.

[lmm]

Huh? I didn't know GWT had any non-UI functionality.