Do early return before checks below.

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 11 matching lines @@
     int counter = 0;
     new Timer.periodic(const Duration(milliseconds: 10), (timer) {
       if (counter == 3) {
         counter = 1024;
         timer.cancel();
         // Wait some more time to be sure callback won't be invoked any
         // more.
         new Timer(const Duration(milliseconds: 30), () {
           replyTo.send('DONE');
         });
+        return;
       }
       assert(counter < 3);
       counter++;
     });
   });
 }
 
 cancellingIsolate() {
   port.receive((msg, replyTo) {
     expect(msg, 'START');
@@ skipping 26 matching lines @@
   });
   test('periodic timer in pure isolate', () {
     expect(spawnFunction(periodicTimerIsolate).call('START'),
            completion('DONE'));
   });
   test('cancellation in pure isolate', () {
     expect(spawnFunction(cancellingIsolate).call('START'),
            completion('DONE'));
   });
 }