rework ArgListContributor to use new task model

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

 // Copyright (c) 2014, 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 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;
 import 'completion_target_test.dart' as completion_target_test;
 import 'dart/test_all.dart' as dart_contributor_tests;
 import 'imported_reference_contributor_test.dart' as imported_test;
 import 'local_declaration_visitor_test.dart' as local_declaration_visitor_test;
 import 'local_reference_contributor_test.dart'
     as local_reference_contributor_test;
 import 'optype_test.dart' as optype_test;
 import 'prefixed_element_contributor_test.dart' as invocation_test;
 import 'uri_contributor_test.dart' as uri_contributor_test;
 
 /// Utility for manually running all tests.
 main() {
   initializeTestEnvironment();
   group('completion', () {
-    arglist_test.main();
     combinator_test.main();
     completion_computer_test.main();
     completion_manager_test.main();
     completion_target_test.main();
     dart_contributor_tests.main();
     imported_test.main();
     invocation_test.main();
     local_declaration_visitor_test.main();
     local_reference_contributor_test.main();
     optype_test.main();
     uri_contributor_test.main();
   });
 }