Flutter 获取图片的高 height 和宽 width

引入头
```
import 'dart:ui' as ui;
```

其次获取图片，主要是用到 ui.Image 里的 addListener 方法。

var imagePath = "";
var image = Image.file(File(imagePath));
Completer<ui.Image> completer = new Completer<ui.Image>();
image.image
    .resolve(new ImageConfiguration())
    .addListener(ImageStreamListener((ImageInfo info, bool _) {
      completer.complete(info.image);
}));

ui.Image info = await completer.future;
int width = info.width;
int height = info.height;
print('-------------->>>${width}x${height}');