Revert "Remove Expect from core library."

tests/corelib/iterable_element_at_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();
 
@@ skipping 18 matching lines @@
   Expect.isTrue(set1.contains(set1.elementAt(2)));
   Expect.throws(() => set1.elementAt(-1), (e) => e is ArgumentError);
   Expect.throws(() => set1.elementAt(3), (e) => e is RangeError);
 
   Expect.throws(() => set2.elementAt("2"),
                 (e) => e is ArgumentError || e is TypeError);
   Expect.throws(() => set2.elementAt(-1), (e) => e is ArgumentError);
   Expect.throws(() => set2.elementAt(0), (e) => e is RangeError);
   Expect.throws(() => set2.elementAt(1), (e) => e is RangeError);
 }