Fix uses of throw/rethrow in tests.

tests/language/stack_trace_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.
 // Dart test program for testing throw statement
 
 import "package:expect/expect.dart";
 
 class MyException {
   const MyException(String message) : message_ = message;
   final String message_;
@@ skipping 52 matching lines @@
     } on MyException catch (exception) {
       throw new MyException("Exception Test for stack trace being printed");;
     }
     return 10;
   }
 
   static int func2() {
     try {
       func();
     } on MyException catch (exception) {
-      throw;
+      rethrow;
     }
     return 10;
   }
 
 }
 
 class StackTraceTest {
   static testMain() {
     Expect.equals(850, Helper.f1(1));
   }
 }
 
 main() {
   StackTraceTest.testMain();
 }