Revert "Remove Expect from core library."

tests/corelib/iterable_to_set_test.dart

 // Copyright (c) 2012, 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() {
   List<int> list1 = <int>[1, 2, 3];
   List<int> list2 = const <int>[4, 4];
   List<String> list3 = <String>[];
   Set<int> set1 = new Set<int>();
   set1..add(11)
       ..add(12)
       ..add(13);
   Set<String> set2 = new Set<String>();
   set2..add("foo")
@@ skipping 31 matching lines @@
   Expect.isTrue(setCopy is Set<String>);
   Expect.isFalse(setCopy is Set<int>);
   Expect.isFalse(identical(setCopy, set2));
 
   setCopy = set3.toSet();
   Expect.setEquals(set3, setCopy);
   Expect.isTrue(setCopy is Set<String>);
   Expect.isTrue(setCopy is Set<int>);
   Expect.isFalse(identical(setCopy, set3));
 }