Remove Expect from core library.

tests/language/top_level_prefixed_declaration_test.dart

 // Copyright (c) 2012, 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 for top level declarations involving an imported type.
 
 library main;
+import "package:expect/expect.dart";
 import "library11.dart" as lib11;
 
 lib11.Library11 variable = null;
 
 lib11.Library11 function() => null;
 
 lib11.Library11 get getter => null;
 
 main() {
   Expect.isTrue(variable == null);
   Expect.isTrue(function() == null);
   Expect.isTrue(getter == null);
 }