更新的解决方案：

随着新版本的flutter，旧解决方案已过时。现在addListener需要ImageStreamListener。

Widget build(BuildContext context) {
    Image image = new Image.network('https://i.stack.imgur.com/lkd0a.png');
    Completer<ui.Image> completer = new Completer<ui.Image>();
    image.image
      .resolve(new ImageConfiguration())
      .addListener(ImageStreamListener(ImageInfo info, bool _) { 
        completer.complete(info.image));
      })
    ...
    ...

原始版本：

如果您已经有一个Image小部件，则可以ImageStream通过调用它来读取resolve它的内容ImageProvider。

import 'dart:ui' as ui;
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(new MaterialApp(
    home: new MyHomePage(),
  ));
}

class MyHomePage extends StatelessWidget {

  Widget build(BuildContext context) {
    Image image = new Image.network('https://i.stack.imgur.com/lkd0a.png');
    Completer<ui.Image> completer = new Completer<ui.Image>();
    image.image
      .resolve(new ImageConfiguration())
      .addListener((ImageInfo info, bool _) => completer.complete(info.image));
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Image Dimensions Example"),
      ),
      body: new ListView(
        children: [
          new FutureBuilder<ui.Image>(
            future: completer.future,
            builder: (BuildContext context, AsyncSnapshot<ui.Image> snapshot) {
              if (snapshot.hasData) {
                return new Text(
                  '${snapshot.data.width}x${snapshot.data.height}',
                  style: Theme.of(context).textTheme.display3,
                );
              } else {
                return new Text('Loading...');
              }
            },
          ),
          image,
        ],
      ),
    );
  }
}