Widget-testing `showDialog`/`AlertDialog` in Flutter

main.dart

import 'package:flutter_test/flutter_test.dart';
import 'package:flutter/material.dart';

const buttonKey = Key("button");
const alertDialogKey = Key("alertDialog");

class MyApp extends StatelessWidget {
  showAppDialog(BuildContext context) async {
    print("Showing app dialog");
    await showDialog(
        context: context,
        builder: (context) {
          return AlertDialog(
            key: alertDialogKey,
            title: const Text(
              "A alert dialog.",
            ),
            icon: const Icon(Icons.catching_pokemon),
            actions: [
              TextButton(
                onPressed: () {
                  Navigator.of(context).pop();
                },
                child: const Text("Oops"),
              ),
            ],
          );
        });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Dialog',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(body: SafeArea(child: Builder(builder: (context) {
        return TextButton(
          key: buttonKey,
          child: const Text("Show dialog"),
          onPressed: () async => await showAppDialog(context),
        );
      }))),
    );
  }
}

void mainTest() {
  testWidgets(
    "When button is pressed, dialog is shown.",
    (tester) async {
      final widget = MyApp();
      await tester.pumpWidget(widget);

      final button = find.byKey(buttonKey);
      expect(button, findsOneWidget);

      await tester.runAsync(() async {
        await tester.tap(button);
//         Or alternatively press then "up":
//         final response = await tester.press(button);
//         await response.up();
      });
      await tester.pumpAndSettle();

      // These all work now
      expect(find.byKey(alertDialogKey), findsOneWidget);
      expect(find.byIcon(Icons.catching_pokemon), findsOneWidget);
      // Still doesn't work, so I'll check for data in the AlertDialog:
      // expect(
      //     find.byElementType(AlertDialog, skipOffstage: false), findsOneWidget);
    },
  );
}

void main() {
  // Set to false to run app so you can interact
  const runTest = true;
  if (runTest) {
    mainTest();   
  } else {
    runApp(MyApp());
  }
}