[native_assets_cli] Add user_defines

[dcharkes]

Bug: #39

Note passing in the user-defines should be done by the Dart and Flutter SDK, they need to decide where to take the command-line arguments, and how to read from the pubspec.yaml.

The NativeAssetsBuildRunner provides a suggestion about where to put user-defines in the pubspec.yaml:

hooks:
  user_defines:
    my_package:
      user_define_key: user_define_value
      user_define_key2:
        foo: bar
    some_other_package:
      user_define_key3: user_define_value3

Moreover, it provides a helper function that can be reused in SDKs after the pubspec yaml is parsed.

Design choices

(According to the discussion on the bug.)

• User-defines are name-spaced per package, we don't want new user-defines to invalidate the cache for other packages.
• User-defines can be provided for any package in the dependency graph.
• If user-defines need to be shared across packages, the common dependency package can export it as metadata.

Test

This change is tested by invoking the build with user-defines, and in the hook failing if the defines are not available.

[github-actions]

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License Headers ✔️

// 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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 84.832% (-0.09%) from 84.923%
when pulling df5af88 on user-defines
into 8a1f836 on main.