rework ArgListContributor to use new task model

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

 // Copyright (c) 2015, 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 analysis_server.src.provisional.completion.completion_dart;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
@@ skipping 56 matching lines @@
 
   /**
    * Return a [Future] that completes with a compilation unit in which
    * all declarations in all scopes containing [target] have been resolved.
    * The [Future] may return `null` if the unit cannot be resolved
    * (e.g. unlinked part file).
    * Any information obtained from [target] prior to calling this method
    * 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);
 }