Remove Expect from core library.

tests/language/reexport_core_test.dart

 // Copyright (c) 2013, 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.
 
 // Tests that exports are handled for libraries loaded prior to the entry point
 // library.
 //
 // This test uses the fact that dart2js loads dart:core before the
 // reexport_core_helper and reexport_core_test libraries and the exports of
 // dart:core is therefore computed before the exports of reexport_core_helper.
 
 library reexport_core_test;
 
+import "package:expect/expect.dart";
 import 'reexport_core_helper.dart' as core;
 
 void main() {
   var o = new Object();
   Expect.isTrue(o is core.Object);
 }