Don't use non-0 Timers for async tests.

tests/lib/async/stream_subscription_cancel_test.dart

Index: tests/lib/async/stream_subscription_cancel_test.dart
diff --git a/tests/lib/async/stream_subscription_cancel_test.dart b/tests/lib/async/stream_subscription_cancel_test.dart
index 3010bc1516d46e83f7fd90ef45595164d0e398e6..d6c6987545fcc04568eaed5f881e8c08d8a37f94 100644
--- a/tests/lib/async/stream_subscription_cancel_test.dart
+++ b/tests/lib/async/stream_subscription_cancel_test.dart
@@ -18,7 +18,7 @@ void main() {
     expect(controller.stream.listen(null).cancel().then((_) => done),
            completion(equals(true)));
 
-    new Timer(const Duration(milliseconds: 10), () {
+    Timer.run(() {
       done = true;
       completer.complete();
     });