Fix event.returnValue warnings in Chrome
Fix event.returnValue warnings in Chrome

No need to worry about adding more code.  Closure compiler ends up
rewriting this as

```
(b.preventDefault?b.preventDefault():b.returnValue=!1,
b.stopPropagation?b.stopPropagation():b.cancelBubble=!0)
```

mousetrap-js/mousetrap,mousetrap-js/mousetrap,ccampbell/mousetrap,ccampbell/mousetrap,PolicyStat/combokeys