Fix gray tiles when tile image is already avaible

[maRci002]

Hello, this PR make sure the loadTileImage's listeners runs after the tile is in _tiles map .

  /// Adds a listener callback that is called whenever a new concrete [ImageInfo]
  /// object is available or an error is reported. If a concrete image is
  /// already available, or if an error has been already reported, this object
  /// will notify the listener synchronously.
  ///
  /// If the [ImageStreamCompleter] completes multiple images over its lifetime,
  /// this listener's [ImageStreamListener.onImage] will fire multiple times.
  ///
  /// {@macro flutter.painting.imageStream.addListener}
  void addListener(ImageStreamListener listener) {

If a concrete image is already available, or if an error has been already reported, this object will notify the listener synchronously.

Since this code:

var key = _tileCoordsToKey(coords);
tile = _tiles[key];
if (null == tile) {
  return;
}

skipped fade-in part, the tiles remained gray. This only happaned if image object was in phone's ram so there wasn't an async callback since image was instantly avaible, however this fix doesn't assume async image load anymore.

closes #707
closes #713
closes #636
closes #608

[maRci002]

How to test, just change flutter_map dependecy to:

  flutter_map:
    git:
      url: https://github.com/fleaflet/flutter_map.git
      ref: 7413bba7b91f6697f7912aa2660465a940d7c9e4

[johnpryan]

(nitpick) This code could use the cascade operator to simplify:

_tiles[tileCoordsToKey] = Tile(
  // ...
)..loadTileImage();

[maRci002]

I tried it but didn't work. I'll give it another try maybe hotreload bugged.

[maRci002]

I retried it didn't work. I think this happens under hood when cascade operator is used.

1. Tile object created
2. loadTileImage method called on Tile
3. Tile added to _tiles[tileCoordsToKey]

With temp variable this happens:

1. Tile object created
2. Tile added to _tiles[tileCoordsToKey]
3. loadTileImage method called on Tile

So even if loadTileImage's callback is synchronous we are sure it is already in _tiles map.