Revert "Remove Expect from core library."

tests/corelib/string_runes_test.dart

 // Copyright (c) 2011, 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() {
   test(String s, List<int> expectedRunes) {
     Runes runes = s.runes;
     Expect.identical(s, runes.string);
 
     // for-in
     var res = [];
     for (int rune in runes) {
       res.add(rune);
     }
@@ skipping 54 matching lines @@
   // Setting position in the middle of a surrogate pair is not allowed.
   var r = new Runes("\u{10000}");
   var it = r.iterator;
   it.moveNext();
   Expect.equals(0x10000, it.current);
 
   // Setting rawIndex inside surrogate pair.
   Expect.throws(() { it.rawIndex = 1; }, (e) => e is Error);
   Expect.throws(() { it.reset(1); }, (e) => e is Error);
 }