Add missing null-tests to async error functions.

tests/lib/async/future_test.dart

 // Copyright (c) 2012, 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.
 
 library future_test;
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 
@@ skipping 659 matching lines @@
     Expect.fail("Shouldn't happen");
   }, onError: (Future f) {
     f.then((v) {
       Expect.equals(42, v);
       asyncEnd();
     });
   });
 }
 
 void testCompleteErrorWithNull() {
+  asyncStart();
   final completer = new Completer<int>();
-  Expect.throws(() => completer.completeError(null));
+  completer.future.catchError((e) {
+    Expect.isTrue(e is NullThrownError);
+    asyncEnd();
+  });
+  completer.completeError(null);
 }
 
 void testChainedFutureValue() {
   final completer = new Completer();
   final future = completer.future;
   asyncStart();
 
   future.then((v) => new Future.value(v * 2))
         .then((v) {
           Expect.equals(42, v);
@@ skipping 119 matching lines @@
       _realFuture.then(action, onError: onError);
   Future catchError(Function onError, {bool test(e)}) =>
       _realFuture.catchError(onError, test: test);
   Future whenComplete(action()) => _realFuture.whenComplete(action);
   Future timeout(Duration timeLimit, {void onTimeout()}) =>
       _realFuture.timeout(timeLimit, onTimeout: onTimeout);
   Stream asStream() => _realFuture.asStream();
   String toString() => "CustomFuture@${_realFuture.hashCode}";
   int get hashCode => _realFuture.hashCode;
 }