Revert IDB optional mode parameter.

tests/html/indexeddb_1_test.dart

 library IndexedDB1Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html' as html;
 import 'dart:indexed_db' as idb;
 
 const String DB_NAME = 'Test';
 const String STORE_NAME = 'TEST';
 const int VERSION = 1;
 
 testReadWrite(key, value, matcher,
               [dbName = DB_NAME,
                storeName = STORE_NAME,
                version = VERSION]) => () {
   var db;
 
   fail(e) {
     guardAsync(() {
       throw new Exception('IndexedDB failure');
     });
   }
 
   createObjectStore(db) {
     var store = db.createObjectStore(storeName);
     expect(store, isNotNull);
   }
 
   step2(e) {
-    var transaction = db.transaction(storeName);
+    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();
       expect(object, matcher);
     }));
     request.onError.listen(fail);
   }
 
   step1() {
@@ skipping 55 matching lines @@
       throw new Exception('IndexedDB failure');
     });
   }
 
   createObjectStore(db) {
     idb.ObjectStore store = db.createObjectStore(storeName);
     expect(store, isNotNull);
   }
 
   step2(e) {
-    idb.Transaction transaction = db.transaction(storeName);
+    idb.Transaction transaction = db.transaction(storeName, 'readonly');
     idb.Request request = transaction.objectStore(storeName).getObject(key);
     request.onSuccess.listen(expectAsync1((e) {
       var object = e.target.result;
       db.close();
       expect(object, matcher);
     }));
     request.onError.listen(fail);
   }
 
   step1() {
@@ skipping 79 matching lines @@
       var db = html.window.indexedDB;
     }, expectation);
   });
 
   // Don't bother with these tests if it's unsupported.
   if (idb.IdbFactory.supported) {
     tests_dynamic();
     tests_typed();
   }
 }