Revert "Remove Expect from core library."

tests/language/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 while statement.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int f1(bool b) {
     while (b)
       return 1;
 
     return 2;
   }
 
   static int f2(bool b) {
     while (b) {
@@ skipping 21 matching lines @@
     Expect.equals(0, Helper.f3(-1));
     Expect.equals(0, Helper.f3(0));
     Expect.equals(1, Helper.f3(1));
     Expect.equals(2, Helper.f3(2));
   }
 }
 
 main() {
   WhileTest.testMain();
 }