Revert "Remove Expect from core library."

tests/language/native_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 which shows how to write self verifying tests
 // and how to print dart objects if needed.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int foo(int i) {
     return i + 10;
   }
 }
 
 class NativeTest {
   static testMain() {
     int i = 10;
     int result = 10 + 10 + 10;
     i = Helper.foo(i + 10);
     print("$i is result.");
     Expect.equals(i, result);
   }
 }
 
 
 main() {
   NativeTest.testMain();
 }