Rewrite async test to make it non-flaky.

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 22 matching lines @@
       }
       assert(counter < 3);
       counter++;
     });
   });
 }
 
 cancellingIsolate() {
   port.receive((msg, replyTo) {
     expect(msg, 'START');
-    final oneshot = new Timer(const Duration(milliseconds: 30), () {
-      fail('Should never be invoked');
-    });
     bool shot = false;
+    var oneshot;
     var periodic;
     periodic = new Timer.periodic(const Duration(milliseconds: 10), (timer) {
       expect(shot, isFalse);
       shot = true;
       expect(timer, same(periodic));
       periodic.cancel();
       oneshot.cancel();
       // Wait some more time to be sure callbacks won't be invoked any
       // more.
       new Timer(const Duration(milliseconds: 50), () {
         replyTo.send('DONE');
       });
     });
+    // We launch the oneshot timer after the periodic timer. Otherwise a
+    // (very long) context switch could make this test flaky: assume the
+    // oneshot timer is created first and then there is a 30ms context switch.
+    // when the periodic timer is scheduled it would execute after the oneshot.
+    oneshot = new Timer(const Duration(milliseconds: 30), () {
+      fail('Should never be invoked');
+    });
   });
 }
 
 main() {
   useHtmlConfiguration();
 
   test('one shot timer in pure isolate', () {
     expect(spawnFunction(oneshotTimerIsolate).call('START'),
            completion('DONE'));
   });
   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'));
   });
 }