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..282d7e9aff70af7dea84c893708851d687928dc6 100644
--- a/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
+++ b/tools/dom/templates/html/dart2js/impl_IDBDatabase.darttemplate
@@ -6,7 +6,7 @@ part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
-  Transaction transaction(storeName_OR_storeNames, String mode) {
+  Transaction transaction(storeName_OR_storeNames, {String mode: 'readonly'}) {

[blois, 2013/02/04 18:21:54]

I would move the default value out of here- in general they are problematic as
they impede overrides. I believe the practical guidance has been to only use
defaults for bools.

Then use an if (mode != null) check to default it (also moving away from ?
operator).

     if (mode != 'readonly' && mode != 'readwrite') {
       throw new ArgumentError(mode);
     }