private <W extends Widget & HasAllDragAndDropHandlers> void verifyDragLeaveEventSink(W w) {
DragLeaveHandlerImpl handler = new DragLeaveHandlerImpl();
w.addDragLeaveHandler(handler);
assertFalse(handler.hasEventFired());
w.fireEvent(new DragLeaveEvent() {
});
assertTrue(handler.hasEventFired());
}