Sort doc-imports according to the directives_ordering rule (flutter#152058)

We're going to enforce sorting doc-imports with the same lint rule as
imports and exports: `directives_ordering`. These three locations need
sorting.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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Author: srawlins

packages/flutter/lib/src/cupertino/page_scaffold.dart

@@ -4,8 +4,8 @@
 
 /// @docImport 'button.dart';
 /// @docImport 'nav_bar.dart';
-/// @docImport 'tab_scaffold.dart';
 /// @docImport 'route.dart';
+/// @docImport 'tab_scaffold.dart';
 library;
 
 import 'package:flutter/foundation.dart';

packages/flutter/lib/src/cupertino/thumb_painter.dart

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/// @docImport 'switch.dart';
 /// @docImport 'slider.dart';
+/// @docImport 'switch.dart';
 library;
 
 import 'package:flutter/painting.dart';

packages/flutter_test/lib/src/image.dart

@@ -4,8 +4,8 @@
 
 /// @docImport 'package:fake_async/fake_async.dart';
 ///
-/// @docImport 'widget_tester.dart';
 /// @docImport 'test_compat.dart';
+/// @docImport 'widget_tester.dart';
 library;
 
 import 'dart:async';