Attempt to make stream-timeout test less flaky.

tests/lib/async/stream_timeout_test.dart

Index: tests/lib/async/stream_timeout_test.dart
diff --git a/tests/lib/async/stream_timeout_test.dart b/tests/lib/async/stream_timeout_test.dart
index ffd25d84d6ead5c3d4c73fd2d1e3ed0ae41646e0..80fe8114b3dcc16ef1ed429d460953745394a239 100644
--- a/tests/lib/async/stream_timeout_test.dart
+++ b/tests/lib/async/stream_timeout_test.dart
@@ -100,8 +100,10 @@ main() {
     Stream tos = c.stream.timeout(halfSec, onTimeout: (_) {
       int elapsed = sw.elapsedMilliseconds;
       if (elapsed > 250) {
-        // This should not happen.
+        // This should not happen, but it does occasionally.
+        // Starving the periodic timer has made the test useless.
         print("Periodic timer of 5 ms delayed $elapsed ms.");
+        return;
       }
       fail("Timeout not prevented by events");
       throw "ERROR";
@@ -125,8 +127,10 @@ main() {
     Stream tos = c.stream.timeout(halfSec, onTimeout: (_) {
       int elapsed = sw.elapsedMilliseconds;
       if (elapsed > 250) {
-        // This should not happen.
+        // This should not happen, but it does occasionally.
+        // Starving the periodic timer has made the test useless.
         print("Periodic timer of 5 ms delayed $elapsed ms.");
+        return;
       }
       fail("Timeout not prevented by errors");
     });