Tail class _tileReady issue

[liuqian5036]

hi,when used CachedNetworkTileProvider to load same url tile,loadTileImage method can case a error, it cause tile image not load.
for example:
first load: "_imageStream.addListener(_listener);" will async exec ,so _addTile method can put tile in _tiles.when _tileReady is ready.
var key = _tileCoordsToKey(coords);
tile = _tiles[key];
if (null == tile) { //it ok ,not entry here,exec load image
return;
};
when widget rebuild again:
because some url, _imageStream.addListener(_listener); already have cache image,so _tileReady run now,but _tiles list is empty,so in _tileReady method,
var key = _tileCoordsToKey(coords);
tile = _tiles[key]; //_tiles is empty,
if (null == tile) { //entry here,exit ,so no load image.
return;
};

am i have some config error?or this is a issue?
please tell me.

[maRci002]

Try #715 fix out.