Revert "Remove Expect from core library."

tests/corelib/iterable_take_while_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];
   List<String> list3 = <String>[];
   Set<int> set1 = new Set<int>();
   set1..add(11)
       ..add(12)
       ..add(13);
   Set set2 = new Set();
 
@@ skipping 106 matching lines @@
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
 
   takeEverything = set2.takeWhile((x) => true);
   it = takeEverything.iterator;
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
 }