Revert IDB optional mode parameter.

tests/html/indexeddb_2_test.dart

 library IndexedDB1Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html' as html;
 import 'dart:indexed_db' as idb;
 import 'dart:collection';
 import 'utils.dart';
 
 // Write and re-read Maps: simple Maps; Maps with DAGs; Maps with cycles.
 
@@ skipping 12 matching lines @@
       throw new Exception('IndexedDB failure');
     });
   }
 
   createObjectStore(db) {
     var store = db.createObjectStore(storeName);
     expect(store, isNotNull);
   }
 
   step2(e) {
-    var transaction = db.transaction(storeName, mode : 'readonly');
+    var transaction = db.transaction(storeName, 'readonly');
     var request = transaction.objectStore(storeName).getObject(key);
     request.onSuccess.listen(expectAsync1((e) {
       var object = e.target.result;
       db.close();
       check(value, object);
     }));
     request.onError.listen(fail);
   }
 
   step1() {
-    var transaction = db.transaction([storeName], mode : 'readwrite');
+    var transaction = db.transaction([storeName], 'readwrite');
     var request = transaction.objectStore(storeName).put(value, key);
     request.onSuccess.listen(expectAsync1(step2));
     request.onError.listen(fail);
   }
 
   initDb(e) {
     db = e.target.result;
     if (version != db.version) {
       // Legacy 'setVersion' upgrade protocol.
       var request = db.setVersion('$version');
@@ skipping 74 matching lines @@
     go('test_DAG', obj2);
     go('test_cycle', obj3);
     go('test_simple_splay', obj4);
     go('const_array_1', const [const [1], const [2]]);
     go('const_array_dag', const [const [1], const [1]]);
     go('array_deferred_copy', [1,2,3, obj3, obj3, 6]);
     go('array_deferred_copy_2', [1,2,3, [4, 5, obj3], [obj3, 6]]);
     go('cyclic_list', cyclic_list);
   }
 }