Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/constant_expressions_test.dart

Index: tests/kernel/unsorted/constant_expressions_test.dart
diff --git a/tests/kernel/unsorted/constant_expressions_test.dart b/tests/kernel/unsorted/constant_expressions_test.dart
index 0ddd9a34a07b46b3c9b32d142fb32edfc72050e8..0d5a3ec39101070dabac887e854391c4847f8ec2 100644
--- a/tests/kernel/unsorted/constant_expressions_test.dart
+++ b/tests/kernel/unsorted/constant_expressions_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';
 
 const int42 = 40 + 2;
 const stringAB = 'a' + 'b';