function getDataFromCustomEvent(nativeEvent) {
	  var detail = nativeEvent.detail;
	  if (typeof detail === 'object' && 'data' in detail) {
	    return detail.data;
	  }
	  return null;
	}
Google Input Tools provides composition data via a CustomEvent,
with the `data` property populated in the `detail` object. If this
is available on the event object, use it. If not, this is a plain
composition event and we have nothing special to extract.

@param {object} nativeEvent
@return {?string}