[code_assets] Add mini_audio example (#2539)

Can play `meow.wav`. 🐈 

Example similar in style to sqlite and #2537. But this interacts with audio, which is a bit harder to unit test. So instead have a `bin/` script.

Author: dcharkes

pkgs/code_assets/example/build/mini_audio/README.md

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+# mini_audio
+
+This project is a minimal, cross-platform demonstration of how to use Dart's
+[build hooks][], with code assets and `dart:ffi` to call a simple function from
+a popular C library. This project uses the `miniaudio` library to play audio
+files.
+
+[build hooks]: https://dart.dev/tools/hooks
+
+## How it works
+
+The native code is built by the [`hook/build.dart`](hook/build.dart), which is
+run by the Dart SDK CLI tools. This build hook compiles the C code from
+[`third_party/miniaudio.c`](third_party/miniaudio.c) into a dynamic library.
+
+The Dart FFI bindings to the C code are generated by
+[`tool/ffigen.dart`](tool/ffigen.dart). FFIgen generates Dart FFI bindings from
+[`third_party/miniaudio.h`](third_party/miniaudio.h) into
+[`lib/src/third_party/miniaudio.g.dart`](lib/src/third_party/miniaudio.g.dart).
+
+The Dart code in [`lib/src/mini_audio.dart`](lib/src/mini_audio.dart) uses the
+generated FFI bindings to provide a Dart API (not exposing any C types).

pkgs/code_assets/example/build/mini_audio/bin/mini_audio.dart

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+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:mini_audio/src/mini_audio.dart';
+
+void main(List<String> args) async {
+  final engine = MiniAudio();
+  if (args.isEmpty) {
+    print('Provide a path to a .wav file.');
+    return;
+  }
+
+  engine.playSound(args.first);
+
+  print('Press Enter to quit...');
+  stdin.readLineSync();
+  engine.uninit();
+}

pkgs/code_assets/example/build/mini_audio/hook/build.dart

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+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. 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:code_assets/code_assets.dart';
+import 'package:hooks/hooks.dart';
+import 'package:logging/logging.dart';
+import 'package:native_toolchain_c/native_toolchain_c.dart';
+
+void main(List<String> args) async {
+  await build(args, (input, output) async {
+    if (input.config.buildCodeAssets) {
+      final cbuilder = CBuilder.library(
+        name: 'miniaudio',
+        assetName: 'src/third_party/miniaudio.g.dart',
+        sources: ['third_party/miniaudio.c'],
+        defines: {
+          if (input.config.code.targetOS == OS.windows)
+            // Ensure symbols are exported in dll.
+            'MA_API': '__declspec(dllexport)',
+        },
+      );
+      await cbuilder.run(
+        input: input,
+        output: output,
+        logger: Logger('')
+          ..level = Level.ALL
+          ..onRecord.listen((record) => print(record.message)),
+      );
+    }
+  });
+}

pkgs/code_assets/example/build/mini_audio/lib/mini_audio.dart

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+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+export 'src/mini_audio.dart';

pkgs/code_assets/example/build/mini_audio/lib/src/mini_audio.dart

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+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+import 'package:ffi/ffi.dart';
+
+import 'third_party/miniaudio.g.dart';
+
+/// A wrapper around the miniaudio engine.
+final class MiniAudio {
+  late final Pointer<ma_engine> _engine;
+
+  /// Initializes the miniaudio engine.
+  MiniAudio() {
+    _engine = malloc();
+    final result = ma_engine_init(nullptr, _engine);
+    if (result != ma_result.MA_SUCCESS) {
+      throw MiniAudioException(
+        'Failed to initialize miniaudio engine: ${result.name}.',
+      );
+    }
+  }
+
+  /// Uninitializes the miniaudio engine and frees resources.
+  void uninit() {
+    ma_engine_uninit(_engine);
+    malloc.free(_engine);
+  }
+
+  /// Plays a sound from the given [filePath].
+  void playSound(String filePath) => using((arena) {
+    final filePath_ = filePath.toNativeUtf8(allocator: arena);
+    final result = ma_engine_play_sound(_engine, filePath_.cast(), nullptr);
+    if (result != ma_result.MA_SUCCESS) {
+      throw MiniAudioException('Failed to play audio: ${result.name}}.');
+    }
+  });
+}
+
+/// An exception that is thrown when an error occurs in the mini_audio library.
+final class MiniAudioException implements Exception {
+  /// The error message.
+  final String message;
+
+  /// Creates a new instance of the exception.
+  MiniAudioException(this.message);
+}