Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/nullable_operator_test.dart

Index: tests/kernel/unsorted/nullable_operator_test.dart
diff --git a/tests/kernel/unsorted/nullable_operator_test.dart b/tests/kernel/unsorted/nullable_operator_test.dart
index 2643a83c9b8e4c457c1f8c403f4191994818f84e..9bf1bcb72c95bf9a3936d1f795703d821f23d62d 100644
--- a/tests/kernel/unsorted/nullable_operator_test.dart
+++ b/tests/kernel/unsorted/nullable_operator_test.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 mkOne() => 1;
 mkNull() => null;