[native_assets_cli] Remove CodeAsset os and architecture getters (#2138)

Bug: #2127

We want to remove these fields as they are redundant with the `CodeConfig`.

This PR does not yet change anything in the protocol, but removes all the uses of the fields.

When rolling this PR into Dart and Flutter, we'll need take the OS and architecture from the `CodeConfig` passed into a hook build.

Author: dcharkes

pkgs/native_assets_builder/test_data/add_asset_link/hook/link.dart

@@ -13,8 +13,8 @@ void main(List<String> arguments) async {
               package: 'add_asset_link',
               name: 'dylib_add_link',
               linkMode: builtDylib.linkMode,
-              os: builtDylib.os,
-              architecture: builtDylib.architecture,
+              os: input.config.code.targetOS,
+              architecture: input.config.code.targetArchitecture,
               file: builtDylib.file,
             ),
           )

pkgs/native_assets_cli/lib/src/code_assets/code_asset.dart

@@ -5,6 +5,7 @@
 import '../config.dart';
 import '../encoded_asset.dart';
 import 'architecture.dart';
+import 'config.dart';
 import 'link_mode.dart';
 import 'os.dart';
 import 'syntax.g.dart' as syntax;
@@ -34,31 +35,30 @@ import 'syntax.g.dart' as syntax;
 /// * Assets which designate symbols present in the target system
 ///   ([DynamicLoadingSystem]), process ([LookupInProcess]), or executable
 ///   ([LookupInExecutable]). These assets do not have a [file].
-/// * Dynamic libraries bundled into the application
-///   ([DynamicLoadingBundled]). These assets must provide a [file] to be
-///   bundled.
+/// * Dynamic libraries bundled into the application ([DynamicLoadingBundled]).
+///   These assets must provide a [file] to be bundled.
 ///
-/// An application is compiled to run on a specific target [os] and
-/// [architecture]. Different targets require different assets, so the package
-/// developer must specify which asset to bundle for which target.
+/// An application is compiled to run on a specific target [CodeConfig.targetOS]
+/// and [CodeConfig.targetArchitecture]. Different targets require different
+/// assets, so the package developer must specify which asset to bundle for
+/// which target.
 ///
 /// An asset has different ways of being accessible in the final application. It
 /// is either brought in "manually" by having the package developer specify a
 /// [file] path of the asset on the current system, it can be part of the Dart
 /// or Flutter SDK ([LookupInProcess]), or it can be already present in the
-/// target system ([DynamicLoadingSystem]). If the asset is bundled
-/// "manually", the Dart or Flutter SDK will take care of copying the asset
-/// [file] from its specified location on the current system into the
-/// application bundle.
+/// target system ([DynamicLoadingSystem]). If the asset is bundled "manually",
+/// the Dart or Flutter SDK will take care of copying the asset [file] from its
+/// specified location on the current system into the application bundle.
 final class CodeAsset {
   /// The id of this code asset.
   final String id;
 
   /// The operating system this asset can run on.
-  final OS os;
+  final OS _os;
 
   /// The architecture this asset can run on.
-  final Architecture architecture;
+  final Architecture _architecture;
 
   /// The link mode for this native code.
   ///
@@ -99,10 +99,11 @@ final class CodeAsset {
   CodeAsset._({
     required this.id,
     required this.linkMode,
-    required this.os,
+    required OS os,
     required this.file,
-    required this.architecture,
-  });
+    required Architecture architecture,
+  }) : _architecture = architecture,
+       _os = os;
 
   factory CodeAsset.fromEncoded(EncodedAsset asset) {
     assert(asset.type == CodeAsset.type);
@@ -128,8 +129,8 @@ final class CodeAsset {
   }) => CodeAsset._(
     id: id ?? this.id,
     linkMode: linkMode ?? this.linkMode,
-    os: os ?? this.os,
-    architecture: architecture ?? this.architecture,
+    os: os ?? _os,
+    architecture: architecture ?? _architecture,
     file: file ?? this.file,
   );
 
@@ -140,28 +141,36 @@ final class CodeAsset {
     }
     return other.id == id &&
         other.linkMode == linkMode &&
-        other.architecture == architecture &&
-        other.os == os &&
+        other._architecture == _architecture &&
+        other._os == _os &&
         other.file == file;
   }
 
   @override
-  int get hashCode => Object.hash(id, linkMode, architecture, os, file);
+  int get hashCode => Object.hash(id, linkMode, _architecture, _os, file);
 
   EncodedAsset encode() {
     final encoding = syntax.NativeCodeAssetEncoding(
-      architecture: architecture.toSyntax(),
+      architecture: _architecture.toSyntax(),
       file: file,
       id: id,
       linkMode: linkMode.toSyntax(),
-      os: os.toSyntax(),
+
+      os: _os.toSyntax(),
     );
     return EncodedAsset(CodeAsset.type, encoding.json);
   }
 
   static const String type = 'native_code';
 }
 
+// These field will be removed in the future, prevent anyone from reading them.
+extension ForValidationOnly on CodeAsset {
+  OS get os => _os;
+
+  Architecture get architecture => _architecture;
+}
+
 extension OSLibraryNaming on OS {
   /// The default dynamic library file name on this os.
   String dylibFileName(String name) {

pkgs/native_assets_cli/lib/src/code_assets/validation.dart

@@ -4,8 +4,14 @@
 
 import 'dart:io';
 
-import '../../code_assets_builder.dart';
+import '../config.dart';
+import '../encoded_asset.dart';
+import '../validation.dart';
+import 'code_asset.dart';
+import 'config.dart';
 import 'link_mode.dart';
+import 'link_mode_preference.dart';
+import 'os.dart';
 import 'syntax.g.dart' as syntax;
 
 Future<ValidationErrors> validateCodeAssetBuildInput(BuildInput input) async =>