Add pkg:mix dependency

[craiglabenz]

Adds mix dependency

[github-actions]

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// Copyright (c) 2024, 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.

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pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/default_dart.dart
pkgs/samples/lib/default_flutter.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/google_ai.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart
pkgs/samples/tool/samples.dart

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[craiglabenz]

It seems I added mix to the wrong pubspec file, as it depends on Flutter but dart_services apparently does not

[parlough]

After chatting separately, I went ahead and added package:mix to the template and regenerated the deps files.

Looks good to me assuming @johnpryan is ok with the additional package support :)

[johnpryan]

LGTM, although I'm not too familiar with how mix uses package:build - do we have a sample app we can use to test and verify that the core functionality works?