Revert "Remove Expect from core library."

tests/corelib/sort_helper.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.
 
 library sort_helper;
-import "package:expect/expect.dart";
 
 class SortHelper {
 
   SortHelper(this.sortFunction, this.compareFunction) {}
 
   void run() {
     testSortIntLists();
     testSortDoubleLists();
   }
 
@@ skipping 126 matching lines @@
 
     for (int i = 0; i < a.length; i++) {
       a[i] = 1.5;
     }
     testSort(a);
   }
 
   Function sortFunction;
   Function compareFunction;
 }