rework ArgListContributor to use new task model

pkg/analysis_server/test/services/completion/dart/completion_contributor_util.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.dart.util;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart' as protocol
     show Element, ElementKind;
@@ skipping 43 matching lines @@
 
   void assertHasParameterInfo(CompletionSuggestion suggestion) {
     expect(suggestion.parameterNames, isNotNull);
     expect(suggestion.parameterTypes, isNotNull);
     expect(suggestion.parameterNames.length, suggestion.parameterTypes.length);
     expect(suggestion.requiredParameterCount,
         lessThanOrEqualTo(suggestion.parameterNames.length));
     expect(suggestion.hasNamedParameters, isNotNull);
   }
 
+  void assertNoSuggestions({CompletionSuggestionKind kind: null}) {
+    if (kind == null) {
+      if (suggestions.length > 0) {
+        failedCompletion('Expected no suggestions', suggestions);
+      }
+      return;
+    }
+    CompletionSuggestion suggestion = suggestions.firstWhere(
+        (CompletionSuggestion cs) => cs.kind == kind,
+        orElse: () => null);
+    if (suggestion != null) {
+      failedCompletion('did not expect completion: $completion\n  $suggestion');
+    }
+  }
+
   void assertNotSuggested(String completion) {
     CompletionSuggestion suggestion = suggestions.firstWhere(
         (CompletionSuggestion cs) => cs.completion == completion,
         orElse: () => null);
     if (suggestion != null) {
       failedCompletion('did not expect completion: $completion\n  $suggestion');
     }
   }
 
   CompletionSuggestion assertSuggest(String completion,
@@ skipping 135 matching lines @@
   }
 
   @override
   void setUp() {
     super.setUp();
     index = createLocalMemoryIndex();
     searchEngine = new SearchEngineImpl(index);
     contributor = createContributor();
   }
 }