Revert "Remove Expect from core library."

tests/corelib/iterable_single_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> list1a = <int>[1];
   List<int> list1b = <int>[1, 2, 3];
   List<int> list1c = <int>[];
   List<int> list2a = const <int>[5];
   List<int> list2b = const <int>[4, 5];
   List<int> list2c = const <int>[];
   Set<int> set1 = new Set<int>();
   set1..add(22);
   Set set2 = new Set();
   set2..add(11)
       ..add(12)
       ..add(13);
   Set set3 = new Set();
 
   Expect.equals(1, list1a.single);
   Expect.throws(() => list1b.single, (e) => e is StateError);
   Expect.throws(() => list1c.single, (e) => e is StateError);
 
   Expect.equals(5, list2a.single);
   Expect.throws(() => list2b.single, (e) => e is StateError);
   Expect.throws(() => list2c.single, (e) => e is StateError);
 
   Expect.equals(22, set1.single);
   Expect.throws(() => set2.single, (e) => e is StateError);
   Expect.throws(() => set3.single, (e) => e is StateError);
 }