Revert "Remove Expect from core library."

tests/corelib/iterable_single_matching_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, 5, 6];
   List<String> list3 = <String>[];
   Set<int> set1 = new Set<int>();
   set1..add(11)
       ..add(12)
       ..add(13);
   Set set2 = new Set();
 
   Expect.equals(2, list1.singleMatching((x) => x.isEven));
   Expect.equals(3, list1.singleMatching((x) => x == 3));
   Expect.throws(() => list1.singleMatching((x) => x.isOdd),
                 (e) => e is StateError);
 
   Expect.equals(6, list2.singleMatching((x) => x == 6));
   Expect.equals(5, list2.singleMatching((x) => x.isOdd));
   Expect.throws(() => list2.singleMatching((x) => x.isEven),
                 (e) => e is StateError);
 
   Expect.throws(() => list3.singleMatching((x) => x == 0),
                 (e) => e is StateError);
 
   Expect.equals(12, set1.singleMatching((x) => x.isEven));
   Expect.equals(11, set1.singleMatching((x) => x == 11));
   Expect.throws(() => set1.singleMatching((x) => x.isOdd));
 
   Expect.throws(() => set2.singleMatching((x) => true), (e) => e is StateError);
 }