Remove Expect from core library.

tests/language/second_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.
 // Second dart test program.
 
+import "package:expect/expect.dart";
+
 class Helper {
   static empty() { }
   static int foo() { return 42; }
 }
 
 class SecondTest {
   static testMain() {
     Helper.empty();
     Expect.equals(42, Helper.foo());
   }
 }
 
 
 main() {
   SecondTest.testMain();
 }