Make mode argument optional for IDB transaction.

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

Index: tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
diff --git a/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate b/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
index cc7154d66db691eaa1b6b5bf5f12d25ccee7b19c..5050e636f40458f2d267f460b518570a859ed4d1 100644
--- a/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
+++ b/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
@@ -6,7 +6,8 @@ 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);
     }