Revert "Remove Expect from core library."

tests/corelib/map_values2_test.dart

 // Copyright (c) 2011, 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 "package:expect/expect.dart";
-
 main() {
   var map1 = <String, int>{ "foo": 42, "bar": 499 };
   var map2 = <String, int>{};
   var map3 = const <String, int>{ "foo": 42, "bar": 499 };
   var map4 = const <String, int>{};
   var map5 = new Map<String, int>();
   map5["foo"] = 43;
   map5["bar"] = 500;
   var map6 = new Map<String, int>();
 
@@ skipping 27 matching lines @@
   Expect.equals(2, map5.values.length);
   Expect.isTrue(map5.values.first == 43 || map5.values.first == 500);
   Expect.isTrue(map5.values.last == 43 || map5.values.first == 500);
   Expect.notEquals(map5.values.first, map5.values.last);
 
   Expect.isTrue(map6.values is Iterable<int>);
   Expect.isFalse(map6.values is Iterable<bool>);
   Expect.isFalse(map6.values is List);
   Expect.equals(0, map6.values.length);
 }