看来Flutter web 1.10 dev，Universal_html软件包是以下临时空缺职位的不错选择dart:html：

import 'dart:convert';
import 'dart:typed_data';

import 'package:flutter/material.dart';
import 'package:universal_html/prefer_universal/html.dart' as html;

class ImagePickerLabPage extends StatefulWidget {
  @override
  _ImagePickerLabPageState createState() => _ImagePickerLabPageState();
}

class _ImagePickerLabPageState extends State<ImagePickerLabPage> {
  String name = '';
  String error;
  Uint8List data;

  pickImage() {
    final html.InputElement input = html.document.createElement('input');
    input
      ..type = 'file'
      ..accept = 'image/*';

    input.onChange.listen((e) {
      if (input.files.isEmpty) return;
      final reader = html.FileReader();
      reader.readAsDataUrl(input.files[0]);
      reader.onError.listen((err) => setState(() {
            error = err.toString();
          }));
      reader.onLoad.first.then((res) {
        final encoded = reader.result as String;
        // remove data:image/*;base64 preambule
        final stripped =
            encoded.replaceFirst(RegExp(r'data:image/[^;]+;base64,'), '');

        setState(() {
          name = input.files[0].name;
          data = base64.decode(stripped);
          error = null;
        });
      });
    });

    input.click();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(name),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.open_in_browser),
        onPressed: () {
          pickImage();
        },
      ),
      body: Center(
        child: error != null
            ? Text(error)
            : data != null ? Image.memory(data) : Text('No data...'),
      ),
    );
  }
}