Fix duplicate entries of elements in a category when re-exported.

If a package has two libraries `foo.dart` and `bar.dart` where
 * `foo.dart` contains `Foo` with an `/// {@category ...}` annotation,
   and,
 * `bar.dart` re-exports `foo.dart`.

Then the category page shall not list `Foo` twice, which is simply a
bug.

This change fixes this issue.

Author: jonasfj

lib/src/model/category.dart

@@ -32,33 +32,33 @@ class Category
   @override
   final DartdocOptionContext config;
 
-  final List<Class> _classes = [];
+  final Set<Class> _classes = {};
 
-  final List<Class> _exceptions = [];
+  final Set<Class> _exceptions = {};
 
   @override
-  final List<TopLevelVariable> constants = [];
+  final Set<TopLevelVariable> constants = {};
 
   @override
-  final List<Extension> extensions = [];
+  final Set<Extension> extensions = {};
 
   @override
-  final List<ExtensionType> extensionTypes = [];
+  final Set<ExtensionType> extensionTypes = {};
 
   @override
-  final List<Enum> enums = [];
+  final Set<Enum> enums = {};
 
   @override
-  final List<ModelFunction> functions = [];
+  final Set<ModelFunction> functions = {};
 
   @override
-  final List<Mixin> mixins = [];
+  final Set<Mixin> mixins = {};
 
   @override
-  final List<TopLevelVariable> properties = [];
+  final Set<TopLevelVariable> properties = {};
 
   @override
-  final List<Typedef> typedefs = [];
+  final Set<Typedef> typedefs = {};
 
   final CategoryDefinition _categoryDefinition;
 

test/end2end/model_special_cases_test.dart

@@ -393,6 +393,25 @@ void main() {
       expect(SubForDocComments.categories.first.isDocumented, isFalse);
       expect(SubForDocComments.displayedCategories, isEmpty);
     });
+
+    test('No duplicate entries', () {
+      final categories = ginormousPackageGraph.localPackages
+          .firstWhere((p) => p.name == 'test_package')
+          .categories;
+      for (final c in categories) {
+        expect(c.classes.length, equals(c.classes.toSet().length));
+        expect(c.exceptions.length, equals(c.exceptions.toSet().length));
+        expect(c.extensions.length, equals(c.extensions.toSet().length));
+        expect(
+            c.extensionTypes.length, equals(c.extensionTypes.toSet().length));
+        expect(c.enums.length, equals(c.enums.toSet().length));
+        expect(c.mixins.length, equals(c.mixins.toSet().length));
+        expect(c.constants.length, equals(c.constants.toSet().length));
+        expect(c.properties.length, equals(c.properties.toSet().length));
+        expect(c.functions.length, equals(c.functions.toSet().length));
+        expect(c.typedefs.length, equals(c.typedefs.toSet().length));
+      }
+    });
   });
 
   group('Package', () {

test/templates/category_test.dart

@@ -125,7 +125,9 @@ extension type ExType(int it) {}
 '''),
         d.file('other.dart', '''
 /// {@category Documented}
-library;      
+library;
+
+export 'lib.dart' show C1, E1;
 '''),
       ],
       files: [
@@ -229,6 +231,15 @@ library;
     );
   });
 
+  test('classes are not duplicated', () async {
+    expect(
+      topicOneLines
+          .where((l) => l.contains('<a href="../lib/C1-class.html">C1</a>')),
+      // Once in the sidebar and once in the main body
+      hasLength(2),
+    );
+  });
+
   test('sidebar contains enums', () async {
     expect(
       topicOneLines,
@@ -240,6 +251,14 @@ library;
     );
   });
 
+  test('enums are not duplicated', () async {
+    expect(
+      topicOneLines.where((l) => l.contains('<a href="../lib/E1.html">E1</a>')),
+      // Once in the sidebar and once in the main body
+      hasLength(2),
+    );
+  });
+
   test('sidebar contains mixins', () async {
     expect(
       topicOneLines,

testing/test_package/lib/src/somelib.dart

@@ -1,5 +1,6 @@
 library reexport.somelib;
 
+/// {@category Unreal}
 class SomeClass {}
 
 class SomeOtherClass {}