Test of CustomScrollViewExampleApp (#152431)

CustomScrollViewExampleApp 
Part of https://github.com/flutter/flutter/issues/130459

The test is checking: 
- if all crucial Widgets are initially visible
- if IconButton click will expand existing SliverList
- if IconButton click and mouse scroll will reveal additional SliverList

dev/bots/check_code_samples.dart

@@ -358,6 +358,5 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/restoration/restoration_mixin.0_test.dart',
   'examples/api/test/widgets/actions/focusable_action_detector.0_test.dart',
   'examples/api/test/widgets/focus_scope/focus_scope.0_test.dart',
-  'examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart',
   'examples/api/test/gestures/pointer_signal_resolver/pointer_signal_resolver.0_test.dart',
 };

examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart

@@ -0,0 +1,47 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/scroll_view/custom_scroll_view.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('What should be visible in the initial state.', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.CustomScrollViewExampleApp());
+
+    expect(find.descendant(
+      of: find.byType(IconButton),
+      matching: find.byIcon(Icons.add),
+    ), findsOne);
+    expect(find.byType(SliverList), findsOne);
+
+    // Initial state should present only "Item: 0" on the SliverList.
+    expect(find.widgetWithText(SliverList, 'Item: 0'), findsOne);
+    expect(find.widgetWithText(SliverList, 'Item: -1'), findsNothing);
+    expect(find.widgetWithText(SliverList, 'Item: 1'), findsNothing);
+});
+
+  testWidgets('Items are added correctly', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.CustomScrollViewExampleApp());
+
+    await tester.tap(find.byType(IconButton));
+    await tester.pump();
+
+    // 'Item: -1' is invisible before scrolling.
+    expect(find.widgetWithText(SliverList, 'Item: -1'), findsNothing);
+    expect(find.widgetWithText(SliverList, 'Item: 1'), findsOne);
+
+    // Scroll the updated screen.
+    await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 50.0));
+    await tester.pump();
+
+    // An additional SliverList appears.
+    expect(find.byType(SliverList), findsExactly(2));
+
+    // All items are visible.
+    expect(find.widgetWithText(SliverList, 'Item: -1'), findsOne);
+    expect(find.widgetWithText(SliverList, 'Item: 0'), findsOne);
+    expect(find.widgetWithText(SliverList, 'Item: 1'), findsOne);
+  });
+}