[flutter_image] Adopt code excerpts in README (flutter#5498)

Updates the README to use a compiled excerpt source for its example of using `NetworkImageWithRetry`.

Part of [flutter#102679](flutter#102679)

Author: mike-v2

packages/flutter_image/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * Fixes image asset link to use image within package.
 * Updates minimum supported SDK version to Flutter 3.10/Dart 3.0.
+* Updates README to improve example of using `NetworkImageWithRetry`.
 
 ## 4.1.9
 

packages/flutter_image/README.md

@@ -7,9 +7,10 @@ network with a retry mechanism.
 
 Example:
 
+<?code-excerpt "example/lib/readme_excerpts.dart (NetworkImageWithRetry)"?>
 ```dart
-var avatar = new Image(
-  image: new NetworkImageWithRetry('http://example.com/avatars/123.jpg'),
+const Image avatar = Image(
+  image: NetworkImageWithRetry('http://example.com/avatars/123.jpg'),
 );
 ```
 

packages/flutter_image/example/lib/readme_excerpts.dart

@@ -0,0 +1,17 @@
+// 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:flutter/widgets.dart';
+import 'package:flutter_image/flutter_image.dart';
+
+/// Demonstrates loading an image for the README.
+Image networkImageWithRetry() {
+// #docregion NetworkImageWithRetry
+  const Image avatar = Image(
+    image: NetworkImageWithRetry('http://example.com/avatars/123.jpg'),
+  );
+// #enddocregion NetworkImageWithRetry
+
+  return avatar;
+}

packages/flutter_image/example/test/readme_excerpts_test.dart

@@ -0,0 +1,22 @@
+// 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:flutter/widgets.dart';
+import 'package:flutter_image/flutter_image.dart';
+import 'package:flutter_image_example/readme_excerpts.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('networkImageWithRetry returns an Image with NetworkImageWithRetry', () {
+    // Ensure that the snippet code runs successfully.
+    final Image result = networkImageWithRetry();
+
+    // It should have a image property of the right type.
+    expect(result.image, isInstanceOf<NetworkImageWithRetry>());
+    // And the NetworkImageWithRetry should have a url property.
+    final NetworkImageWithRetry networkImage =
+        result.image as NetworkImageWithRetry;
+    expect(networkImage.url, equals('http://example.com/avatars/123.jpg'));
+  });
+}

script/configs/temp_exclude_excerpt.yaml

@@ -8,7 +8,6 @@
 - css_colors
 - espresso
 - extension_google_sign_in_as_googleapis_auth
-- flutter_image
 - go_router_builder
 - google_sign_in/google_sign_in
 - image_picker_for_web