rework ArgListContributor to use new task model

pkg/analysis_server/test/services/completion/dart/test_all.dart

Index: pkg/analysis_server/test/services/completion/dart/test_all.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/test_all.dart b/pkg/analysis_server/test/services/completion/dart/test_all.dart
index dfa7512f76d003986774c2b0047663201c498f12..156ce7ade3178f7083481074db38d72bd75e8d4f 100644
--- a/pkg/analysis_server/test/services/completion/dart/test_all.dart
+++ b/pkg/analysis_server/test/services/completion/dart/test_all.dart
@@ -7,6 +7,7 @@ library test.services.completion.dart;
 import 'package:unittest/unittest.dart';
 
 import '../../../utils.dart';
+import 'arglist_contributor_test.dart' as arglist_test;
 import 'common_usage_sorter_test.dart' as common_usage_test;
 import 'inherited_contributor_test.dart' as inherited_contributor_test;
 import 'keyword_contributor_test.dart' as keyword_test;
@@ -15,6 +16,7 @@ import 'keyword_contributor_test.dart' as keyword_test;
 main() {
   initializeTestEnvironment();
   group('dart/completion', () {
+    arglist_test.main();
     common_usage_test.main();
     inherited_contributor_test.main();
     keyword_test.main();