Fix uses of throw/rethrow in tests.

tests/language/try_catch3_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 program for testing try/catch statement without any exceptions
 // being thrown.
 // VMOptions=--optimization-counter-threshold=10
 
 import "package:expect/expect.dart";
 
 abstract class TestException {
@@ skipping 63 matching lines @@
       }
       try {
         j += 80;
       } on MyException catch (e) {
         i = 500;
         print(e.getMessage());
       }
     } on MyParameterizedException<String, TestException> catch (e, trace) {
       i = 800;
       trace.printStackTrace(e);
-      throw;
+      rethrow;
     } on MyException catch (exception) {
       i = 100;
       print(exception.getMessage());
     } on TestException catch (e, trace) {
       i = 200;
       trace.printStackTrace(e);
     } finally {
       i = 900;
     }
     return i;
@@ skipping 16 matching lines @@
   static testMain() {
     Expect.equals(900, Helper.test1(1));
   }
 }
 
 main() {
   for (var i = 0; i < 20; i++) {
     TryCatchTest.testMain();
   }
 }