Revert "Remove Expect from core library."

tests/language/null_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.
 // Second dart test program.
 
-import "package:expect/expect.dart";
-
 class NullTest {
   static int foo(var obj) {
     Expect.equals(null, obj);
   }
 
   static bool compareToNull(var value) {
     return null == value;
   }
 
   static bool compareWithNull(var value) {
@@ skipping 27 matching lines @@
     Expect.isFalse(val is !Object);
 
     return 0;
   }
 }
 
 
 main() {
   NullTest.testMain();
 }