(TBR) Fix more failing tests

pkg/analysis_server/test/integration/analysis/error_test.dart

Index: pkg/analysis_server/test/integration/analysis/error_test.dart
diff --git a/pkg/analysis_server/test/integration/analysis/error_test.dart b/pkg/analysis_server/test/integration/analysis/error_test.dart
index 847f8956dbb3af327f7c73ec0b465e06b833dd47..f2abb51eefcad29faab98b9ecf520e954a730ef5 100644
--- a/pkg/analysis_server/test/integration/analysis/error_test.dart
+++ b/pkg/analysis_server/test/integration/analysis/error_test.dart
@@ -63,11 +63,11 @@ abstract class C extends B {
     expect(
         allErrorMessages,
         contains(
-            "The class 'C' cannot be used as a mixin because it extends a class other than Object"));
+            "The class 'C' can't be used as a mixin because it extends a class other than Object."));
     expect(
         allErrorMessages,
         contains(
-            "The class 'C' cannot be used as a mixin because it references 'super'"));
+            "The class 'C' can't be used as a mixin because it references 'super'."));
   }
 
   test_super_mixins_enabled() async {