Sort doc-imports according to the directives_ordering rule (#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.

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we expect every widget to implement].
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above.
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[test-exempt].
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Fixes] where supported.
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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';