你可以这样做：

class Functions  {
  void onListen() {}
}

class MockFunctions extends Mock implements Functions {}

void main() {
  test("onListen is called once when first listener is registered", () {
    final functions = MockFunctions();

    when(functions.onListen()).thenReturn(null);

    final bloc = EntityListBloc(onListen: functions.onListen);

    verify(functions.onListen()).called(1);
  });
}

公认的答案是正确的，但它并不代表您可能希望用mock或fake替换顶级函数的真实场景。This article解释了如何在依赖注入组合中包含顶级函数，以便可以用mock替换这些函数。
您可以像这样编写依赖注入，并指向顶级函数，例如launchUrl和ioc_container。

IocContainerBuilder compose() => IocContainerBuilder(
      allowOverrides: true,
    )
      ..addSingletonService<LaunchUrl>(
        (url, {mode, webOnlyWindowName, webViewConfiguration}) async =>
            launchUrl(
          url,
          mode: mode ?? LaunchMode.platformDefault,
          webViewConfiguration:
              webViewConfiguration ?? const WebViewConfiguration(),
          webOnlyWindowName: webOnlyWindowName,
        ),
      )
      ..add((container) => MyApp(launchUrl: container<LaunchUrl>()));

然后，你可以使用这里的答案中提到的技巧来用Mocktail进行模拟。

import 'package:fafsdfsdf/main.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';
import 'package:url_launcher/url_launcher.dart';
import 'package:flutter/material.dart';

class LaunchMock extends Mock {
  Future<bool> call(
    Uri url, {
    LaunchMode? mode,
    WebViewConfiguration? webViewConfiguration,
    String? webOnlyWindowName,
  });
}

void main() {
  testWidgets('Test Url Launch', (tester) async {
    //These allow default values
    registerFallbackValue(LaunchMode.platformDefault);
    registerFallbackValue(const WebViewConfiguration());

    //Create the mock
    final mock = LaunchMock();
    when(() => mock(
          flutterDevUri,
          mode: any(named: 'mode'),
          webViewConfiguration: any(named: 'webViewConfiguration'),
          webOnlyWindowName: any(named: 'webOnlyWindowName'),
        )).thenAnswer((_) async => true);

    final builder = compose()
      //Replace the launch function with a mock
      ..addSingletonService<LaunchUrl>(mock);

    await tester.pumpWidget(
      builder.toContainer()<MyApp>(),
    );

    //Tap the icon
    await tester.tap(
      find.byIcon(Icons.favorite),
    );

    await tester.pumpAndSettle();

    verify(() => mock(flutterDevUri)).called(1);
  });
}