loadImage method
Loads a single image given the image's url
and adds it to the ImageMap.
Implementation
Future<ui.Image> loadImage(String url) async {
ImageStream stream = new AssetImage(url, bundle: _bundle).resolve(ImageConfiguration.empty);
Completer<ui.Image> completer = new Completer<ui.Image>();
void listener(ImageInfo frame, bool synchronousCall) {
final ui.Image image = frame.image;
_images[url] = image;
completer.complete(image);
stream.removeListener(listener);
}
stream.addListener(listener);
return completer.future;
}