Revert "Remove Expect from core library."

tests/language/call_through_null_getter_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.
 
-import "package:expect/expect.dart";
-
 // Tests that we can call functions through getters which return null.
 
 const TOP_LEVEL_NULL = null;
 
 var topLevel;
 
 class CallThroughNullGetterTest {
 
   static void testMain() {
     testTopLevel();
@@ skipping 45 matching lines @@
   A() { }
   var field;
   get getter { return field; }
   method() { return field; }
 
 }
 
 main() {
   CallThroughNullGetterTest.testMain();
 }