Revert "Remove Expect from core library."

tests/corelib/string_split_reg_exp_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() {
   var list = "a b c".split(new RegExp(" "));
   Expect.equals(3, list.length);
   Expect.equals("a", list[0]);
   Expect.equals("b", list[1]);
   Expect.equals("c", list[2]);
 
   list = "adbdc".split(new RegExp("[dz]"));
   Expect.equals(3, list.length);
   Expect.equals("a", list[0]);
@@ skipping 21 matching lines @@
   Expect.equals("", list[0]);
   Expect.equals("", list[1]);
   Expect.equals("", list[2]);
   Expect.equals("", list[3]);
 
   list = "aaa".split(new RegExp(r"a$"));
   Expect.equals(2, list.length);
   Expect.equals("aa", list[0]);
   Expect.equals("", list[1]);
 }