Remove Expect from core library.

tests/language/closure4_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.
 
+import "package:expect/expect.dart";
+
 // Dart2js failed when a declared function was captured inside itself.
 
 foo(f) => f(499);
 
 main() {
   fun(x) {
     if (x < 3) {
       return foo((x) => fun(x));
     } else {
       return x;
     }
   }
   Expect.equals(499, fun(499));
 }