Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/as_is_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 class literals, is, and as expressions.
 
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 class C {}
 
 class D extends C {}
 
 test0() => C;
 test1() => D;
 
 main() {
   var c = new C();
   var d = new D();
   Expect.isTrue(test0() == C);
   Expect.isTrue(test1() == D);
   Expect.isTrue(c is C);
   Expect.isTrue(c is! D);
   Expect.isTrue(d is C);
   Expect.isTrue(d is D);
   Expect.isTrue(c as C == c);
   Expect.isTrue(d as C == d);
   Expect.isTrue(d as D == d);
 }