fix showing duplicate implementors

Author: keertip

lib/src/model.dart

@@ -25,7 +25,7 @@ void _addToImplementors(Class c) {
     _implementors.putIfAbsent(key, () => []);
     List list = _implementors[key];
 
-    if (!list.contains(implClass)) {
+    if (!list.any((l) => l.element == c.element)) {
       list.add(implClass);
     }
   }

test/model_test.dart

@@ -286,6 +286,10 @@ void main() {
       expect(Apple.mixins, hasLength(0));
     });
 
+    test('implementors', () {
+      expect(Apple.implementors, hasLength(1));
+    });
+
     test('mixins not private', () {
       expect(F.mixins, hasLength(0));
     });

test_package/lib/fake.dart

@@ -43,6 +43,7 @@ library fake;
 import 'dart:async';
 
 import 'example.dart';
+export 'example.dart' show B;
 
 /// Useful for annotations.
 class Annotation {