Fix uses of throw/rethrow in tests.

tests/language/final_used_in_try_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";
 
 main() {
   while (true) {
     final a = 'fff'.substring(1, 2);
     try {
       Expect.equals('f', a);
     } catch (e) {
-      throw;
+      rethrow;
     }
     break;
   }
 }