Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/closures_test.dart

Index: tests/kernel/unsorted/closures_test.dart
diff --git a/tests/kernel/unsorted/closures_test.dart b/tests/kernel/unsorted/closures_test.dart
index da1279404f4b985d19a6f0b2a2efc298ddcc452f..df3430f07a15777144f3b461a6198a220e1bed12 100644
--- a/tests/kernel/unsorted/closures_test.dart
+++ b/tests/kernel/unsorted/closures_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';
 
 class Base {
   int constant1000() => 1000;