Add missing null-tests to async error functions.

tests/lib/async/stream_controller_async_test.dart

Index: tests/lib/async/stream_controller_async_test.dart
diff --git a/tests/lib/async/stream_controller_async_test.dart b/tests/lib/async/stream_controller_async_test.dart
index 43a7ca7a4606d88bc369898083555ae1b5b47a8a..8d61ef9605054309212dfe64676bb9b2377f6ee8 100644
--- a/tests/lib/async/stream_controller_async_test.dart
+++ b/tests/lib/async/stream_controller_async_test.dart
@@ -380,7 +380,7 @@ testRethrow() {
       Stream s = streamErrorTransform(c.stream, (e) { throw error; });
       s.listen((_) { Expect.fail("unexpected value"); }, onError: expectAsync(
           (e) { Expect.identical(error, e); }));
-      c.addError(null);
+      c.addError("SOME ERROR");
       c.close();
     });
   }