Minor tweak to indexeddb_3 test transaction handling

tests/html/indexeddb_3_test.dart

 library IndexedDB3Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:async';
 import 'dart:html' as html;
 import 'dart:indexed_db';
 
 // Read with cursor.
 
 const String DB_NAME = 'Test3';
 const String STORE_NAME = 'TEST';
 const int VERSION = 1;
 
 
 Future<Database> createAndOpenDb() {
   return html.window.indexedDB.deleteDatabase(DB_NAME).then((_) {
     return html.window.indexedDB.open(DB_NAME, version: VERSION,
         onUpgradeNeeded: (e) {
           var db = e.target.result;
           db.createObjectStore(STORE_NAME);
         });
   });
 }
 
 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);
   for (var i = 1; i < 100; ++i) {
     future = future.then((_) => write(i));
   }
 
   // Chain on the DB so we return it at the end.
   return future.then((_) => db);
 }
@@ skipping 65 matching lines @@
     });
     test('readAll1', () {
       return readAllViaCursor(db);
     });
 
     test('readAll2', () {
       return readAllReversedViaCursor(db);
     });
   }
 }