Remove Expect from core library.

tests/language/library_prefixes_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.
 
 library LibraryPrefixesTest.dart;
+import "package:expect/expect.dart";
 import "library_prefixes.lib";
 
 class LibraryPrefixesTest {
   static testMain() {
     LibraryPrefixes.main((a, b) { Expect.equals(a, b); });
   }
 }
 
 main() {
   LibraryPrefixesTest.testMain();
 }