Revert "Remove Expect from core library."

tests/corelib/map_keys2_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 = { "foo": 42, "bar": 499 };
   var map2 = {};
   var map3 = const { "foo": 42, "bar": 499 };
   var map4 = const {};
   var map5 = new Map<String, int>();
   map5["foo"] = 43;
   map5["bar"] = 500;
   var map6 = new Map<String, bool>();
 
   Expect.isTrue(map1.keys is Iterable<String>);
   Expect.isFalse(map1.keys is Iterable<bool>);
 
   Expect.isTrue(map2.keys is Iterable<String>);
   Expect.isFalse(map2.keys is Iterable<bool>);
 
   Expect.isTrue(map3.keys is Iterable<String>);
   Expect.isFalse(map3.keys is Iterable<bool>);
 
   Expect.isTrue(map4.keys is Iterable<String>);
   Expect.isFalse(map4.keys is Iterable<bool>);
 
   Expect.isTrue(map5.keys is Iterable<String>);
   Expect.isFalse(map5.keys is Iterable<bool>);
 
   Expect.isTrue(map6.keys is Iterable<String>);
   Expect.isFalse(map6.keys is Iterable<bool>);
 }