如何知道一个图像是否在Flutter的内存中？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: ImageWidget(),
        ),
      ),
    );
  }
}

class ImageWidget extends StatefulWidget {
  @override
  _ImageWidgetState createState() => _ImageWidgetState();
}

class _ImageWidgetState extends State<ImageWidget> {
  ImageStream _imageStream;
  ImageInfo _imageInfo;

  @override
  void initState() {
    super.initState();
    loadImage();
  }

  @override
  void dispose() {
    _imageStream.removeListener(imageListener);
    super.dispose();
  }

  void loadImage() {
    final ImageProvider imageProvider = AssetImage('assets/image.png');
    _imageStream = imageProvider.resolve(ImageConfiguration.empty);
    _imageStream.addListener(imageListener);
  }

  void imageListener(ImageInfo image, bool synchronousCall) {
    setState(() {
      _imageInfo = image;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Text(_imageInfo != null ? 'Image is loaded' : 'Image is not loaded');
  }
}