Revert "Remove Expect from core library."

tests/corelib/string_split_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";
-
 class StringSplitTest {
   static testMain() {
     var list = "a b c".split(" ");
     Expect.equals(3, list.length);
     Expect.equals("a", list[0]);
     Expect.equals("b", list[1]);
     Expect.equals("c", list[2]);
 
     list = "adbdc".split("d");
     Expect.equals(3, list.length);
@@ skipping 22 matching lines @@
     Expect.equals("", list[0]);
     Expect.equals("", list[1]);
     Expect.equals("", list[2]);
     Expect.equals("", list[3]);
   }
 }
 
 main() {
   StringSplitTest.testMain();
 }