Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/closures_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';
 
 class Base {
   int constant1000() => 1000;
 }
 
 class Foo extends Base {
   final int base;
   Foo(this.base);
 
   nestedAdderFunction(a, b, c, d, e) {
@@ skipping 40 matching lines @@
     funs.add(() => i);
   }
   Expect.isTrue((funs[0]() + funs[1]() + funs[2]()) == 3);
 
   var funs2 = [];
   for (var i in [0, 1, 2]) {
     funs2.add(() => i);
   }
   Expect.isTrue((funs2[0]() + funs2[1]() + funs2[2]()) == 3);
 }