Add HTML test with two dart scripts on one page.

tests/html/scripts_test_dart.dart

 // Copyright (c) 2014, 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.
 
 library ScriptsTestDart;
 import 'dart:html';
 import 'dart:async';
 
 main() {
   window.postMessage('squid', '*');
   window.postMessage('tiger', '*');  // Unexpected message OK.
   new Timer(new Duration(seconds: 1), () {
     window.postMessage('squid', '*');  // Duplicate message OK.
     window.postMessage('sea urchin', '*');
   });
-}
+}