Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/invocation_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 invocations.
 
-import 'expect.dart';
+import 'package:expect/expect.dart';
 
 test0(x) {
   Expect.isTrue(x == 'argument0');
   return 'return0';
 }
 
 class C0 {
   static test1(x) {
     Expect.isTrue(x == 'argument1');
     return 'return1';
@@ skipping 13 matching lines @@
   }
 }
 
 main() {
   Expect.isTrue(test0('argument0') == 'return0');
   Expect.isTrue(C0.test1('argument1') == 'return1');
   Expect.isTrue(new C1().test2('argument2') == 'return2');
   var c = new C2.test3('argument3');
   Expect.isTrue(c is C2);
 }