Cleanup cross imports (flutter#181807)

Remove material from widgets tests:

- reassemble_test.dart
- page_storage_test.dart

Part of flutter#177415
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Author: yeltayev22

dev/bots/check_tests_cross_imports.dart

@@ -144,7 +144,6 @@ class TestsCrossImportChecker {
     'packages/flutter/test/widgets/navigator_replacement_test.dart',
     'packages/flutter/test/widgets/implicit_animations_test.dart',
     'packages/flutter/test/widgets/default_text_editing_shortcuts_test.dart',
-    'packages/flutter/test/widgets/page_storage_test.dart',
     'packages/flutter/test/widgets/sliver_main_axis_group_test.dart',
     'packages/flutter/test/widgets/color_filter_test.dart',
     'packages/flutter/test/widgets/semantics_merge_test.dart',
@@ -211,7 +210,6 @@ class TestsCrossImportChecker {
     'packages/flutter/test/widgets/scroll_activity_test.dart',
     'packages/flutter/test/widgets/tap_region_test.dart',
     'packages/flutter/test/widgets/lookup_boundary_test.dart',
-    'packages/flutter/test/widgets/reassemble_test.dart',
     'packages/flutter/test/widgets/html_element_view_test.dart',
     'packages/flutter/test/widgets/navigator_test.dart',
     'packages/flutter/test/widgets/text_semantics_test.dart',

packages/flutter/test/widgets/page_storage_test.dart

@@ -2,17 +2,19 @@
 // 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/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import 'widgets_app_tester.dart';
+
 void main() {
   testWidgets('PageStorage read and write', (WidgetTester tester) async {
     const Key builderKey = PageStorageKey<String>('builderKey');
     late StateSetter setState;
     var storedValue = 0;
 
     await tester.pumpWidget(
-      MaterialApp(
+      TestWidgetsApp(
         home: StatefulBuilder(
           key: builderKey,
           builder: (BuildContext context, StateSetter setter) {
@@ -40,7 +42,7 @@ void main() {
     var storedValue = 0;
 
     Widget buildWidthKey(Key key) {
-      return MaterialApp(
+      return TestWidgetsApp(
         home: StatefulBuilder(
           key: key,
           builder: (BuildContext context, StateSetter setter) {

packages/flutter/test/widgets/reassemble_test.dart

@@ -2,12 +2,14 @@
 // 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/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import 'widgets_app_tester.dart';
+
 void main() {
   testWidgets('reassemble does not crash', (WidgetTester tester) async {
-    await tester.pumpWidget(const MaterialApp(home: Text('Hello World')));
+    await tester.pumpWidget(const TestWidgetsApp(home: Text('Hello World')));
     await tester.pump();
     tester.binding.reassembleApplication();
     await tester.pump();