Minor tweak to indexeddb_3 test transaction handling

tests/html/indexeddb_3_test.dart

Index: tests/html/indexeddb_3_test.dart
diff --git a/tests/html/indexeddb_3_test.dart b/tests/html/indexeddb_3_test.dart
index f3027e79dc0959023c591971940904612c5250b6..d3030dcd632582525ca4b69ab22141e894af5412 100644
--- a/tests/html/indexeddb_3_test.dart
+++ b/tests/html/indexeddb_3_test.dart
@@ -25,8 +25,8 @@ Future<Database> createAndOpenDb() {
 Future<Database> writeItems(Database db) {
   Future<Object> write(index) {
     var transaction = db.transaction(STORE_NAME, 'readwrite');
-    return transaction.objectStore(STORE_NAME).put('Item $index', index)
-        .then((_) => transaction.onComplete.first);
+    transaction.objectStore(STORE_NAME).put('Item $index', index);
+    return transaction.completed;
   }
 
   var future = write(0);