This is a simple example to test a StreamBuilder widget, which data are from http request.

streambuilder_widget_test.dart

// This is a basic Flutter widget test.
// To perform an interaction with a widget in your test, use the WidgetTester utility that Flutter
// provides. For example, you can send tap and scroll gestures. You can also use WidgetTester to
// find child widgets in the widget tree, read text, and verify that the values of widget properties
// are correct.

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:rxdart/rxdart.dart';
import 'package:http/http.dart' as http;
void main() {

  Widget snapshotText(BuildContext context, AsyncSnapshot<String> snapshot) {
    print(snapshot.toString());
    return new Text(snapshot.toString(), textDirection: TextDirection.ltr);
  }
  Future<Null> eventFiring(WidgetTester tester) async {
    await tester.pump(Duration.zero);
  }
  // create a data stream
  final _dataStreamSubject = BehaviorSubject<String>();
  Stream<String>_dataStream = _dataStreamSubject.stream;
  void refresh() async {
    http.Response response = await http.get(
      "https://jsonplaceholder.typicode.com/todos/1"
    );
    if (response.statusCode == 200) {
      _dataStreamSubject.add("success");
    }else {
      _dataStreamSubject.add("failed");
    }
  }
  
  testWidgets("StreamBuilder from HTTP Request", (WidgetTester tester) async {
    await tester.pumpWidget(new StreamBuilder<String>(
      stream:_dataStream,
      builder: snapshotText
    ));
    await refresh();
    eventFiring(tester);
    expect(find.text("AsyncSnapshot<String>(ConnectionState.active, true, null)"), findsOneWidget);
  });
}

在 testWidgets 方法里面无法添加异步请求方法