I'm not sure I understand - are you extending ToolButton? Your UiBinder doesn't list a ToolButton instance, so I'm not sure how that comes into the picture here.
Neither ToolButton (nor its superclass IconButton) have a field called style, just a constructor with an argument named style - and that isn't a constructor that is called from UiBinder.
It does have a static field called style, but that would only possibly be an issue if you are using uibinder to extend ToolButton instead of using an appearance, as should be done. Is that the use case you are after?