flutter/packages/unit/test/widget/flex_test.dart

import 'package:flutter/widgets.dart';
import 'package:test/test.dart';

import 'widget_tester.dart';

void main() {
  test('Can hit test flex children of stacks', () {
    testWidgets((WidgetTester tester) {
      bool didReceiveTap = false;
      tester.pumpWidget(
        new Container(
          decoration: const BoxDecoration(
            backgroundColor: const Color(0xFF00FF00)
          ),
          child: new Stack(<Widget>[
            new Positioned(
              top: 10.0,
              left: 10.0,
              child: new Column(<Widget>[
                new GestureDetector(
                  onTap: () {
                    didReceiveTap = true;
                  },
                  child: new Container(
                    decoration: const BoxDecoration(
                      backgroundColor: const Color(0xFF0000FF)
                    ),
                    width: 100.0,
                    height: 100.0,
                    child: new Center(
                      child: new Text('X')
                    )
                  )
                )
              ])
            )
          ])
        )
      );

      tester.tap(tester.findText('X'));
      expect(didReceiveTap, isTrue);
    });
  });
}