Attempt to make stream-timeout test less flaky.

tests/lib/async/stream_timeout_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:async";
 import "package:unittest/unittest.dart";
 
 main() {
   const ms5 = const Duration(milliseconds: 5);
   const halfSec = const Duration(milliseconds: 500);
@@ skipping 82 matching lines @@
       expect(s, null);
     })).listen((v){ fail("Unexpected event"); });
   });
 
   test("events prevent timeout", () {
     Stopwatch sw = new Stopwatch();
     StreamController c = new StreamController();
     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";
     });
     tos.listen((v) { expect(v, 42);}, onDone: expectAsync0((){}));
     int ctr = 200;  // send this many events at 5ms intervals. Then close.
     new Timer.periodic(ms5, (timer) {
       sw.reset();
       c.add(42);
       if (--ctr == 0) {
         timer.cancel();
         c.close();
       }
     });
     sw.start();
   });
 
   test("errors prevent timeout", () {
     Stopwatch sw = new Stopwatch();
     StreamController c = new StreamController();
     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");
     });
     tos.listen((_) {},
       onError: (e, s) {
         expect(e, "ERROR");
       },
       onDone: expectAsync0((){}));
     int ctr = 200;  // send this many error events at 5ms intervals. Then close.
     new Timer.periodic(ms5, (timer) {
@@ skipping 22 matching lines @@
       fail("Timeout not prevented by close");
     });
     var subscription = tos.listen((_) {}, onDone: expectAsync0((){}));
     subscription.pause();
     new Timer(halfSec, () {
       c.close();
       subscription.resume();
     });
   });
 }