[native_assets_cli] Expose tagged union tag in syntax (#2141)

Bug: #2132

Take the `Asset.type`s from the syntax.

(This will prevent any misspellings when using `EncodedAsset.type` as a String in `native_assets_cli`.)

Author: dcharkes

pkgs/hooks/tool/generate_syntax.dart

@@ -45,6 +45,7 @@ void main(List<String> args) {
             'HookOutput',
             'LinkOutput',
           ],
+          visbleUnionTagValues: ['Asset'],
         ).analyze();
     final textDumpFile = File.fromUri(
       packageUri.resolve(

pkgs/json_syntax_generator/lib/src/generator/normal_class_generator.dart

@@ -27,6 +27,7 @@ class ClassGenerator {
     buffer.writeln('''
 class $className $extendsString {
 ''');
+    buffer.writeln(_generateTag());
     buffer.writeln(_generateFields());
     buffer.writeln(_generateJsonFactory());
     buffer.writeln(_generateJsonConstructor());
@@ -41,6 +42,16 @@ class $className $extendsString {
 ''');
   }
 
+  /// If this is a tagged union value, expose the tag.
+  String _generateTag() {
+    if (classInfo.superclass?.visibleTaggedUnion != true) return '';
+    final tagProperty = classInfo.superclass!.taggedUnionProperty;
+    final tagValue = classInfo.taggedUnionValue;
+    return '''
+static const ${tagProperty}Value = '$tagValue';
+''';
+  }
+
   String _generateFields() {
     if (classInfo.superclass != null) {
       // The super class already has the required fields.

pkgs/json_syntax_generator/lib/src/model/class_info.dart

@@ -40,6 +40,11 @@ class NormalClassInfo extends ClassInfo {
   /// Only set in the sub classes.
   final String? taggedUnionValue;
 
+  /// If the tagged union tags are available in the generated API.
+  ///
+  /// Only set in the parent class.
+  final bool visibleTaggedUnion;
+
   bool get isTaggedUnion =>
       taggedUnionProperty != null || taggedUnionValue != null;
 
@@ -52,6 +57,7 @@ class NormalClassInfo extends ClassInfo {
     this.taggedUnionProperty,
     this.taggedUnionValue,
     this.extraValidation = const [],
+    this.visibleTaggedUnion = false,
   }) : super() {
     superclass?.subclasses.add(this);
     if (taggedUnionValue != null) {
@@ -77,6 +83,7 @@ $propertiesString
   ],
   taggedUnionProperty: $taggedUnionProperty,
   taggedUnionValue: $taggedUnionValue,
+  visibleTaggedUnion: $visibleTaggedUnion,
   extraValidation: [
 $extraValidationString
   ],

pkgs/json_syntax_generator/lib/src/parser/schema_analyzer.dart

@@ -44,11 +44,29 @@ class SchemaAnalyzer {
   /// Generate public setters for these class names.
   final List<String> publicSetters;
 
+  /// For subtypes of these classes, the union tag values are exposed.
+  ///
+  /// For example, if `Asset.type` is `NativeCodeAsset.typeValue`, then the
+  /// asset is a native code asset. Listing `Asset` in [visbleUnionTagValues]
+  /// will add `static const typeValue = 'native_code';`:
+  ///
+  /// ```dart
+  /// class NativeCodeAsset extends Asset {
+  ///   static const typeValue = 'native_code';
+  /// }
+  ///
+  /// class Asset {
+  ///   String? get type => _reader.get<String?>('type');
+  /// }
+  /// ```
+  final List<String> visbleUnionTagValues;
+
   SchemaAnalyzer(
     this.schema, {
     this.capitalizationOverrides = const {},
     this.classSorting,
     this.publicSetters = const [],
+    this.visbleUnionTagValues = const [],
   });
 
   /// Accumulator for all classes during the analysis.
@@ -164,6 +182,7 @@ class SchemaAnalyzer {
       taggedUnionValue: taggedUnionValue,
       taggedUnionProperty:
           schemas.generateSubClasses ? properties.single.name : null,
+      visibleTaggedUnion: visbleUnionTagValues.contains(typeName),
       extraValidation: extraValidation,
     );
     _classes[typeName] = classInfo;

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

@@ -171,7 +171,7 @@ extension CodeAssetType on CodeAsset {
   // TODO(https://github.com/dart-lang/native/issues/2132): Change to be
   // namespaced by package name. (We'll temporarily need to support both the
   // old a new type.)
-  static const String type = 'native_code';
+  static const String type = syntax.NativeCodeAsset.typeValue;
 }
 
 extension EncodedCodeAsset on EncodedAsset {

pkgs/native_assets_cli/lib/src/code_assets/syntax.g.dart

@@ -815,6 +815,8 @@ class MacOSCodeConfig {
 }
 
 class NativeCodeAsset extends Asset {
+  static const typeValue = 'native_code';
+
   NativeCodeAsset.fromJson(super.json, {super.path}) : super._fromJson();
 
   NativeCodeAsset({

pkgs/native_assets_cli/lib/src/data_assets/data_asset.dart

@@ -84,7 +84,7 @@ extension DataAssetType on DataAsset {
   // TODO(https://github.com/dart-lang/native/issues/2132): Change to be
   // namespaced by package name. (We'll temporarily need to support both the
   // old a new type.)
-  static const String type = 'data';
+  static const String type = syntax.DataAsset.typeValue;
 }
 
 extension EncodedDataAsset on EncodedAsset {

pkgs/native_assets_cli/lib/src/data_assets/syntax.g.dart

@@ -50,6 +50,8 @@ class Asset {
 }
 
 class DataAsset extends Asset {
+  static const typeValue = 'data';
+
   DataAsset.fromJson(super.json, {super.path}) : super._fromJson();
 
   DataAsset({