Revert "Remove Expect from core library."

tests/language/for_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.
 // Dart test program for testing for statement.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int f1() {
     for (;;) return 1;
   }
 
   static int f2(var n) {
     int i = 0;
     for (; i < n; i++);
     return i;
   }
@@ skipping 31 matching lines @@
     Expect.equals(0, Helper.f4(0));
     Expect.equals(1, Helper.f4(1));
     Expect.equals(6, Helper.f4(6));
     Expect.equals(6, Helper.f4(10));
   }
 }
 
 main() {
   ForTest.testMain();
 }