paging.dart export issue

1.0.0-alpha4

export 文件修改

invalidate failed we dont chang the list now

invalidate failed we dont change the list now, so keep listview not redraw

add onResultInitialFailed

when initial load failed need complete invalidate future

make invalidate return Future

RefreshIndicator or other widget may want invalidate return Future ,so make invalidate return Future

1.0.0-alpha

first alpha version