rework ArgListContributor to use new task model

pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart

Index: pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
index ebd551836620326bf8cfadd2f6d3c7796ffdb6c1..816894d639106146447ccdb805d7928ecb3b8e0d 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
@@ -74,4 +74,13 @@ abstract class DartCompletionRequest extends CompletionRequest {
    * should be discarded as it may have changed.
    */
   Future<CompilationUnit> resolveDeclarationsInScope();
+
+  /**
+   * Return a [Future] that completes when the element associated with
+   * the given [identifier] is available or if the identifier cannot be resolved
+   * (e.g. unknown identifier, completion aborted, etc).
+   * Any information obtained from [target] prior to calling this method
+   * should be discarded as it may have changed.
+   */
+  Future resolveIdentifier(SimpleIdentifier identifier);
 }