[cross_file] adopt code excerpts in README (#5347)

Updates the README to use a compiled excerpt source for its example of instantiating an `XFile`.

Part of [flutter/flutter#102679](https://github.com/flutter/flutter/issues/102679)

packages/cross_file/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 0.3.3+7
 
+* Updates README to improve example of instantiating an XFile.
 * Updates minimum supported SDK version to Flutter 3.10/Dart 3.0.
 
 ## 0.3.3+6

packages/cross_file/README.md

@@ -10,18 +10,17 @@ access the file and its metadata.
 
 Example:
 
+<?code-excerpt "example/lib/readme_excerpts.dart (Instantiate)"?>
 ```dart
-import 'package:cross_file/cross_file.dart';
-
-final file = XFile('assets/hello.txt');
+final XFile file = XFile('assets/hello.txt');
 
 print('File information:');
 print('- Path: ${file.path}');
 print('- Name: ${file.name}');
 print('- MIME type: ${file.mimeType}');
 
-final fileContent = await file.readAsString();
-print('Content of the file: ${fileContent}');  // e.g. "Moto G (4)"
+final String fileContent = await file.readAsString();
+print('Content of the file: $fileContent');
 ```
 
 You will find links to the API docs on the [pub page](https://pub.dev/packages/cross_file).

packages/cross_file/example/assets/hello.txt

@@ -0,0 +1 @@
+Hello, world!

packages/cross_file/example/lib/readme_excerpts.dart

@@ -0,0 +1,24 @@
+// Copyright 2013 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.
+
+// ignore_for_file: avoid_print
+
+import 'package:cross_file/cross_file.dart';
+
+/// Demonstrate instantiating an XFile for the README.
+Future<XFile> instantiateXFile() async {
+  // #docregion Instantiate
+  final XFile file = XFile('assets/hello.txt');
+
+  print('File information:');
+  print('- Path: ${file.path}');
+  print('- Name: ${file.name}');
+  print('- MIME type: ${file.mimeType}');
+
+  final String fileContent = await file.readAsString();
+  print('Content of the file: $fileContent');
+  // #enddocregion Instantiate
+
+  return file;
+}

packages/cross_file/example/pubspec.yaml

@@ -0,0 +1,18 @@
+name: cross_file_example
+description: Demonstrates how to use cross files.
+publish_to: none
+
+environment:
+  sdk: ">=3.0.0 <4.0.0"
+
+dependencies:
+  cross_file:
+    # When depending on this package from a real application you should use:
+    #   cross_file: ^x.y.z
+    # See https://dart.dev/tools/pub/dependencies#version-constraints
+    # The example app is bundled with the plugin so we use a path dependency on
+    # the parent directory to use the current plugin's version.
+    path: ../
+
+dev_dependencies:
+  test: ^1.24.0

packages/cross_file/example/test/readme_excerpts_test.dart

@@ -0,0 +1,23 @@
+// Copyright 2013 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:cross_file/cross_file.dart';
+import 'package:cross_file_example/readme_excerpts.dart';
+import 'package:test/test.dart';
+
+const bool kIsWeb = bool.fromEnvironment('dart.library.js_util');
+
+void main() {
+  test('instantiateXFile loads asset file', () async {
+    // Ensure that the snippet code runs successfully.
+    final XFile xFile = await instantiateXFile();
+    // It should have a nonempty path and name.
+    expect(xFile.path, allOf(isNotNull, isNotEmpty));
+    expect(xFile.name, allOf(isNotNull, isNotEmpty));
+
+    // And the example file should have contents.
+    final String fileContent = await xFile.readAsString();
+    expect(fileContent, allOf(isNotNull, isNotEmpty));
+  }, skip: kIsWeb);
+}

packages/cross_file/pubspec.yaml

@@ -2,7 +2,7 @@ name: cross_file
 description: An abstraction to allow working with files across multiple platforms.
 repository: https://github.com/flutter/packages/tree/main/packages/cross_file
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+cross_file%22
-version: 0.3.3+6
+version: 0.3.3+7
 
 environment:
   sdk: ">=3.0.0 <4.0.0"

script/configs/temp_exclude_excerpt.yaml

@@ -5,7 +5,6 @@
 
 # TODO(stuartmorgan): Remove everything from this list. See
 # https://github.com/flutter/flutter/issues/102679
-- cross_file
 - css_colors
 - espresso
 - extension_google_sign_in_as_googleapis_auth