Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/rethrow_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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';
 
 testNormalRethrow() {
   var x = 0;
   try {
     try {
       throw x++;
     } catch (e) {
       Expect.isTrue(e == 0);
       x++;
       rethrow;
@@ skipping 42 matching lines @@
     x++;
   }
   Expect.isTrue(x == 5);
 }
 
 main() {
   testNormalRethrow();
   testNormalRethrow2();
   testRethrowWithinTryRethrow();
 }