Fix test for latest analysis (#543)

protoc_plugin/test/mixin_test.dart

diff --git a/protoc_plugin/test/mixin_test.dart b/protoc_plugin/test/mixin_test.dart
index e4b6641..16bc720 100644
--- a/protoc_plugin/test/mixin_test.dart
+++ b/protoc_plugin/test/mixin_test.dart
@@ -12,8 +12,8 @@
     });
 
     test('is a Mixin1', () {
-      expect(proto is Mixin1, isTrue);
-      expect(proto is Mixin2, isFalse);
+      expect(proto, isA<Mixin1>());
+      expect(proto, isNot(isA<Mixin2>()));
     });
 
     test('implements interface defined by mixins', () {
@@ -59,9 +59,9 @@
     });
 
     test('is both Mixin1 (from parent) and Mixin3', () {
-      expect(proto is Mixin1, isTrue);
-      expect(proto is Mixin2, isFalse);
-      expect(proto is Mixin3, isTrue);
+      expect(proto, isA<Mixin1>());
+      expect(proto, isNot(isA<Mixin2>()));
+      expect(proto, isA<Mixin3>());
     });
   });
 }