Add integration test for asset transformation feature (#145715)

In service of https://github.com/flutter/flutter/issues/143348

This adds a simple integration test for the new asset transformation feature.

packages/flutter_tools/test/integration.shard/asset_transformation_test.dart

@@ -0,0 +1,153 @@
+// Copyright 2014 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:file_testing/file_testing.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/base/io.dart';
+
+import '../src/common.dart';
+import 'test_data/project.dart';
+import 'test_utils.dart';
+
+final class _AssetTransformationTestProject extends Project {
+  @override
+  final String pubspec = '''
+name: test
+environment:
+  sdk: '>=3.2.0-0 <4.0.0'
+dependencies:
+  flutter:
+    sdk: flutter
+dev_dependencies:
+  capitalizer_transformer:
+    path: ./capitalizer_transformer
+flutter:
+  assets:
+    - path: assets/text_asset.txt
+      transformers:
+        - package: capitalizer_transformer
+''';
+
+  @override
+  final String main = '''
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: Text('Hello, World!'),
+        ),
+      ),
+    );
+  }
+}
+''';
+}
+
+Future<void> main() async {
+  testWithoutContext('asset is transformed when declared with a transformation', () async {
+    final Directory tempProjectDirectory = createResolvedTempDirectorySync(
+      'asset_transformation_test.',
+    );
+
+    try {
+      _setUpCapitalizerTransformer(tempProjectDirectory);
+      await _AssetTransformationTestProject().setUpIn(tempProjectDirectory);
+      tempProjectDirectory.childDirectory('assets').childFile('text_asset.txt')
+        ..createSync(recursive: true)
+        ..writeAsStringSync('abc');
+
+      final String flutterBin = fileSystem.path.join(
+        getFlutterRoot(),
+        'bin',
+        'flutter',
+      );
+      final ProcessResult result = await processManager.run(
+        <String>[
+          flutterBin,
+          'build',
+          'web',
+        ],
+        workingDirectory: tempProjectDirectory.path,
+      );
+
+      expect(result.exitCode, 0, reason: result.stderr as String);
+
+      final File asset = fileSystem.file(
+        fileSystem.path.join(
+          tempProjectDirectory.path,
+          'build',
+          'web',
+          'assets',
+          'assets',
+          'text_asset.txt',
+        ),
+      );
+
+      expect(asset, exists);
+
+      expect(
+        asset.readAsStringSync(),
+        equals('ABC'),
+        reason:
+          "The original contents of the asset (which should be 'abc') should "
+          "have been transformed to 'ABC' by the capitalizer_transformer as "
+          'configured in the pubspec.',
+      );
+    } finally {
+      tryToDelete(tempProjectDirectory);
+    }
+  });
+}
+
+void _setUpCapitalizerTransformer(Directory projectDir) {
+  final Directory targetDir = projectDir.childDirectory('capitalizer_transformer');
+  targetDir.createSync(recursive: true);
+
+  targetDir.childFile('pubspec.yaml')
+    ..createSync()
+    ..writeAsStringSync('''
+name: capitalizer_transformer
+version: 1.0.0
+
+environment:
+  sdk: '>=3.2.0-0 <4.0.0'
+
+dependencies:
+  args: ^2.4.2
+''');
+
+  targetDir.childDirectory('bin').childFile('capitalizer_transformer.dart')
+    ..createSync(recursive: true)
+    ..writeAsStringSync('''
+import 'dart:io';
+
+import 'package:args/args.dart';
+
+void main(List<String> args) {
+  final ArgParser parser = ArgParser()
+    ..addOption('input')
+    ..addOption('output');
+
+  final ArgResults parsedArgs = parser.parse(args);
+
+  final String inputFilePath = parsedArgs['input'] as String;
+  final String outputFilePath = parsedArgs['output'] as String;
+
+  final String input = File(inputFilePath).readAsStringSync();
+  File(outputFilePath)
+    ..createSync(recursive: true)
+    ..writeAsStringSync(input.toUpperCase());
+}
+''');
+}