Revert "Remove Expect from core library."

tests/language/do_while_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 do while statement.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int f1(bool b) {
     do return 1;
     while (b);
     return 2;
   }
 
   static int f2(bool b) {
     do {
       return 1;
@@ skipping 36 matching lines @@
     Expect.equals(1, Helper.f5(0));
     Expect.equals(1, Helper.f5(1));
     Expect.equals(2, Helper.f5(2));
     Expect.equals(3, Helper.f5(3));
   }
 }
 
 main() {
   DoWhileTest.testMain();
 }