Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/loop_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.
 
 // Tests of loops.
 
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 fact(n) {
   var f = 1;
   while (n > 0) {
     f *= n;
     --n;
   }
   return f;
 }
 
@@ skipping 64 matching lines @@
   Expect.isTrue(fib(6) == 8);
   Expect.isTrue(fib(7) == 13);
   Expect.isTrue(fib(42) == 267914296);
   Expect.isTrue(fib(3.14159) == 3);
 
   test0();
   test1();
   test2();
   test3();
 }