Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/closures_regression_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';
 
 f(fun) => fun();
 class A {
   identity(arg) {
     return f(() {
       print(this);
       return f(() {
         return this;
       });
     });
   }
 }
 
 main() {
   var a = new A();
   Expect.isTrue(identical(a.identity(42), a));
 }