(TBR) Fix failing tests

pkg/analyzer_cli/test/super_mixin_test.dart

Index: pkg/analyzer_cli/test/super_mixin_test.dart
diff --git a/pkg/analyzer_cli/test/super_mixin_test.dart b/pkg/analyzer_cli/test/super_mixin_test.dart
index 86060051f20ff371e27a64bd186f06aee39c1654..f8902890512529909074263e3108a7a0bbf87c65 100644
--- a/pkg/analyzer_cli/test/super_mixin_test.dart
+++ b/pkg/analyzer_cli/test/super_mixin_test.dart
@@ -45,11 +45,11 @@ void main() {
       expect(
           stdout,
           contains(
-              "[error] The class 'C' cannot be used as a mixin because it extends a class other than Object"));
+              "[error] The class 'C' can't be used as a mixin because it extends a class other than Object"));
       expect(
           stdout,
           contains(
-              "[error] The class 'C' cannot be used as a mixin because it references 'super'"));
+              "[error] The class 'C' can't be used as a mixin because it references 'super'"));
       expect(stdout, contains('2 errors found.'));
       expect(errorSink.toString(), '');
     });