Make OptionElement construtor arguments named optional parameters.

tests/html/htmloptionscollection_test.dart

 // Copyright (c) 2012, 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 HTMLOptionsCollectionTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
@@ skipping 15 matching lines @@
     expect(optionsCollection[2].value, equals('2'));
 
     expect(optionsCollection[0].text, equals('Option0'));
     expect(optionsCollection[1].text, equals('Option1'));
     expect(optionsCollection[2].text, equals('Option2'));
 
     expect(() { optionsCollection[0] = 1; }, throws);
 
     // OPTIONALS optionsCollection[0] = new OptionElement(value: '42', data: 'Option42');
     expect(() {
-      optionsCollection[0] = new OptionElement('Option42', '42');
+      optionsCollection[0] = new OptionElement(data: 'Option42', value: '42');
     }, throws);
   });
 }