code: public static void setEnabled(Element element, boolean enabled) {
          element.setPropertyBoolean("disabled", !enabled);
  	setStyleName(element, "disabled", !enabled);
      }
docs: It's enough to just set the disabled attribute on the
  element, but we want to also add a "disabled" class so that we can
  style it.
  
  At some point we'll just be able to use .button:disabled,
  but that doesn't work in IE8-