Implement rethrow statement

tests/language/rethrow_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();
 }
@@ skipping 11 matching lines @@
   }
 
   void testRethrowPastUncaught() {
     try {
       try {
         try {
           throwException();
           Expect.fail("Should have thrown an exception");
         } catch (e) {
           Expect.equals(true, identical(e, currentException));
-          throw;
+          rethrow;
           Expect.fail("Should have thrown an exception");
         }
       } on OtherException catch (e) {
         Expect.fail("Should not have caught OtherException");
       }
     } catch (e) {
       Expect.equals(true, identical(e, currentException));
     }
   }
 
   void testRethrow() {
     try {
       try {
         throwException();
         Expect.fail("Should have thrown an exception");
       } catch (e) {
         Expect.equals(true, identical(e, currentException));
-        throw;
+        rethrow;
         Expect.fail("Should have thrown an exception");
       }
     } catch (e) {
       Expect.equals(true, identical(e, currentException));
     }
   }
 }
 
 main() {
   RethrowTest t = new RethrowTest();
   t.testRethrow();
   t.testRethrowPastUncaught();
 }