Fix a typo.

tests/html/async_test.dart

 library async_test;
 
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:html';
 
 oneshotTimerIsolate() {
@@ skipping 49 matching lines @@
 }
 
 main() {
   useHtmlConfiguration();
 
   test('one shot timer in pure isolate', () {
     expect(spawnFunction(oneshotTimerIsolate).call('START'),
            completion('DONE'));
   });
   test('periodic timer in pure isolate', () {
-    expect(spawnFunction(oneshotTimerIsolate).call('START'),
+    expect(spawnFunction(periodicTimerIsolate).call('START'),
            completion('DONE'));
   });
   test('cancellation in pure isolate', () {
     expect(spawnFunction(cancellingIsolate).call('START'),
            completion('DONE'));
   });
 }