Make mode argument optional for IDB transaction.

tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
-  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').
     return _transaction(storeName_OR_storeNames, mode);
   }
 
   @JSName('transaction')
   Transaction _transaction(stores, mode) native;
 
 $!MEMBERS}