Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/breakable_statement_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';
 
 testFor() {
   int current;
   for (int i = 0; i < 100; i++) {
     current = i;
     if (i > 41) break;
   }
   Expect.isTrue(current == 42);
 }
 
@@ skipping 56 matching lines @@
 }
 
 main() {
   testFor();
   testWhile();
   testDoWhile();
   testLabledBreakOutermost();
   testLabledBreakMiddle();
   testLabledBreakInner();
 }