Fix uses of throw/rethrow.

tests/language/full_stacktrace2_test.dart

 // (c) 2013, 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";
 
 void func1() {
   throw new Exception("Test full stacktrace");
 }
 void func2() {
   func1();
 }
 void func3() {
   try {
     func2();
   } on Object catch(e, s) {
     var fullTrace = s.toString();
     Expect.isTrue(fullTrace.contains("func1"));
     Expect.isTrue(fullTrace.contains("func2"));
     Expect.isTrue(fullTrace.contains("func3"));
     Expect.isTrue(fullTrace.contains("func4"));
     Expect.isTrue(fullTrace.contains("func5"));
     Expect.isTrue(fullTrace.contains("func6"));
     Expect.isTrue(fullTrace.contains("func7"));
     Expect.isTrue(fullTrace.contains("main"));
 
-    throw;  // This is a rethrow.
+    rethrow;  // This is a rethrow.
   }
 }
 int func4() {
   func3();
   return 1;
 }
 int func5() {
   try {
     func4();
   } on Object catch(e, s) {
@@ skipping 14 matching lines @@
   return 1;
 }
 int func7() {
   func6();
   return 1;
 }
 main() {
   var i = func7();
   Expect.equals(1, i);
 }