Add missing null-tests to async error functions.

tests/lib/async/stream_controller_async_test.dart

 // Copyright (c) 2011, 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.
 
 // Test the basic StreamController and StreamController.broadcast.
 library stream_controller_async_test;
 
 import 'dart:async';
 import 'dart:isolate';
 import "package:expect/expect.dart";
@@ skipping 362 matching lines @@
       c.close();
     });
   }
 
   testStreamError(name, streamErrorTransform) {
     test("rethrow-$name-error", () {
       StreamController c = new StreamController();
       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();
     });
   }
 
   testFuture(name, streamValueTransform) {
     test("rethrow-$name-value", () {
       StreamController c = new StreamController();
       Future f = streamValueTransform(c.stream, (v) { throw error; });
       f.then((v) { Expect.fail("unreachable"); },
              onError: expectAsync((e) { Expect.identical(error, e); }));
@@ skipping 369 matching lines @@
   testRethrow();
   testBroadcastController();
   testAsBroadcast();
   testSink(sync: true, broadcast: false, asBroadcast: false);
   testSink(sync: true, broadcast: false, asBroadcast: true);
   testSink(sync: true, broadcast: true, asBroadcast: false);
   testSink(sync: false, broadcast: false, asBroadcast: false);
   testSink(sync: false, broadcast: false, asBroadcast: true);
   testSink(sync: false, broadcast: true, asBroadcast: false);
 }