Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/closures_regression_test.dart

Index: tests/kernel/unsorted/closures_regression_test.dart
diff --git a/tests/kernel/unsorted/closures_regression_test.dart b/tests/kernel/unsorted/closures_regression_test.dart
index ac45e1bc4caf5dba4544555da463be3b0783577c..51c18d305c39410c75c412d508cbe301f0d85c10 100644
--- a/tests/kernel/unsorted/closures_regression_test.dart
+++ b/tests/kernel/unsorted/closures_regression_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';
 
 f(fun) => fun();
 class A {