Dart closures are automatically converted to JS closures when sent across the border. You can just do this:
ID3.callMethod('loadTags', ["filename.mp3", () {
var tags = ID3.callMethod('getAllTags', ["filename.mp3"]);
var artistTag = tags['artist'];
var titleTag = tags['title'];
if (artistTag != null) {
artist.text = artistTag;
}
if (titleTag != null) {
track.text = titleTag;
}
},
new js.JsObject.jsify({'dataReader': id3FileReader})
]);