Language tests updated to use const instead of final.

tests/language/interface_constants_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.
 
 interface Constants {
-  static final int FIVE = 5;
+  static const int FIVE = 5;
 }
 
 class InterfaceConstantsTest {
   InterfaceConstantsTest() {}
 
   static void testMain() {
     Expect.equals(5, Constants.FIVE);
   }
 }
 
 main() {
   InterfaceConstantsTest.testMain();
 }