rework ArgListContributor to use new task model

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

Index: pkg/analysis_server/test/services/completion/test_all.dart
diff --git a/pkg/analysis_server/test/services/completion/test_all.dart b/pkg/analysis_server/test/services/completion/test_all.dart
index f6e8e8234768f3dd38a033842b2b43d5342a5a8a..85150282c9fea75a775210cc350da9e531e9cd9e 100644
--- a/pkg/analysis_server/test/services/completion/test_all.dart
+++ b/pkg/analysis_server/test/services/completion/test_all.dart
@@ -7,7 +7,6 @@ library test.services.completion;
 import 'package:unittest/unittest.dart';
 
 import '../../utils.dart';
-import 'arglist_contributor_test.dart' as arglist_test;
 import 'combinator_contributor_test.dart' as combinator_test;
 import 'completion_computer_test.dart' as completion_computer_test;
 import 'completion_manager_test.dart' as completion_manager_test;
@@ -25,7 +24,6 @@ import 'uri_contributor_test.dart' as uri_contributor_test;
 main() {
   initializeTestEnvironment();
   group('completion', () {
-    arglist_test.main();
     combinator_test.main();
     completion_computer_test.main();
     completion_manager_test.main();