Rewrite async test to make it non-flaky.

tests/html/async_test.dart

Index: tests/html/async_test.dart
diff --git a/tests/html/async_test.dart b/tests/html/async_test.dart
index 1d7768f878c1d2c23a3d751856ecc4ccbefc6265..616258cd17dbd607711a83d11d2be774843a361c 100644
--- a/tests/html/async_test.dart
+++ b/tests/html/async_test.dart
@@ -40,10 +40,8 @@ periodicTimerIsolate() {
 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);
@@ -57,6 +55,13 @@ cancellingIsolate() {
         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');
+    });
   });
 }