Make mode argument optional for IDB transaction.

sdk/lib/indexed_db/dart2js/indexed_db_dart2js.dart

 library dart.dom.indexed_db;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:_js_helper' show Creates, Returns, JSName, Null;
 import 'dart:_foreign_helper' show JS;
 // 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.
@@ skipping 188 matching lines @@
 // BSD-style license that can be found in the LICENSE file.
 
 
 @DomName('IDBDatabase')
 @SupportedBrowser(SupportedBrowser.CHROME)
 @SupportedBrowser(SupportedBrowser.FIREFOX, '15')
 @SupportedBrowser(SupportedBrowser.IE, '10')
 @Experimental
 class Database extends EventTarget native "*IDBDatabase" {
 
-  Transaction transaction(storeName_OR_storeNames, String mode) {
+  Transaction transaction(storeName_OR_storeNames, {String mode}) {
+    if (mode == null) mode = 'readonly';
     if (mode != 'readonly' && mode != 'readwrite') {
       throw new ArgumentError(mode);
     }
 
     // TODO(sra): Ensure storeName_OR_storeNames is a string or List<String>,
     // and copy to JavaScript array if necessary.
 
     // Try and create a transaction with a string mode.  Browsers that expect a
     // numeric mode tend to convert the string into a number.  This fails
     // silently, resulting in zero ('readonly').
@@ skipping 705 matching lines @@
 }
 // 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.
 
 
 @DocsEditable
 @DomName('IDBAny')
 class _IDBAny native "*IDBAny" {
 }