Remove custom expect.dart from Kernel tests

tests/kernel/unsorted/mapliteral_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';
 
 main() {
   var map = {
     'k1': 'v1',
     'k2': 'v2',
     1: 2,
     1.5 : 1.2,
     3: 3.14,
   };
 
   Expect.isTrue(map.length == 5);
 
   map['foo'] = 'bar';
 
   Expect.isTrue(map['k1'] == 'v1');
   Expect.isTrue(map['k2'] == 'v2');
   Expect.isTrue(map[1] == 2);
   Expect.isTrue(map[1.5] == 1.2);
   Expect.isTrue(map[3] == 3.14);
   Expect.isTrue(map['foo'] == 'bar');
   Expect.isTrue(map.length == 6);
 }