Revert "Remove Expect from core library."

tests/language/void_type_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 type checks involving the void type.
 
-import "package:expect/expect.dart";
-
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
     return true;
   }
 }
 
@@ skipping 59 matching lines @@
 main() {
   test(0, f, false);
   test(1, f_null, false);
   test(2, f_1, true);
   test(3, f_dyn_null, false);
   test(4, f_dyn_1, true);
   test(5, f_f, false);
 }