Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/super_mixin_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 {
   var field;
 
   method(x) {
     print(x);
     return x;
   }
 
   set setter(x) {
@@ skipping 14 matching lines @@
 
 class Sub extends Base with Mixin {
 }
 
 main() {
   var object = new Sub();
   Expect.isTrue(object.method('x') == 'xMixin');
   object.setter = 'y';
   Expect.isTrue(object.field == 'yMixin');
 }