Revert "Remove Expect from core library."

tests/corelib/const_list_literal_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";
-
 // Test that a final list literal is not expandable nor modifiable.
 
 class ConstListLiteralTest {
 
   static void testMain() {
     var list = const [4, 2, 3];
     Expect.equals(3, list.length);
 
     var exception = null;
     try {
@@ skipping 53 matching lines @@
     // correctly.
     int x = 0;
     list.forEach((e) { x += e; });
     Expect.equals(9, x);
   }
 }
 
 main() {
   ConstListLiteralTest.testMain();
 }