rework ArgListContributor to use new task model

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

Index: pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart b/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
index 226e9ac9c4f501a1e8c335519a3d08a5422e1e6d..7794c29552bd599b9bcf7fa1dd184fea40b4f887 100644
--- a/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
+++ b/pkg/analysis_server/test/services/completion/dart/completion_contributor_util.dart
@@ -61,6 +61,21 @@ abstract class DartCompletionContributorTest extends AbstractContextTest {
     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,