pkg/unittest: support up to 6 arguments in expectAsync[Until]

pkg/unittest/test/unittest_middle_exception_test.dart

Index: pkg/unittest/test/unittest_middle_exception_test.dart
diff --git a/pkg/unittest/test/unittest_middle_exception_test.dart b/pkg/unittest/test/unittest_middle_exception_test.dart
index d405500c57d05476bbd634172ccb99aab6034427..ba7751470b30046bbe22f6dd27f360ce13fb44e3 100644
--- a/pkg/unittest/test/unittest_middle_exception_test.dart
+++ b/pkg/unittest/test/unittest_middle_exception_test.dart
@@ -15,7 +15,7 @@ var testFunction = (_) {
   test('testOne', () { expect(true, isTrue); });
   test('testTwo', () { expect(true, isFalse); });
   test('testThree', () {
-    var done = expectAsync0((){});
+    var done = expectAsync(() {});
     _defer(() {
       expect(true, isTrue);
       done();