refactor ContributionSorter to use new API

pkg/analysis_server/lib/src/services/completion/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.services.completion.completion_dart;
 
 import 'package:analysis_server/completion/completion_core.dart';
 import 'package:analysis_server/completion/completion_dart.dart';
+import 'package:analysis_server/completion/dart/completion_target.dart';
 import 'package:analysis_server/src/services/completion/completion_core.dart';
 import 'package:analyzer/src/generated/ast.dart';
 
 /**
  * The information about a requested list of completions within a Dart file.
  */
 class DartCompletionRequestImpl extends CompletionRequestImpl
     implements DartCompletionRequest {
   /**
    * The compilation unit in which the completion was requested.
    */
   final CompilationUnit unit;
 
   /**
    * A flag indicating whether the compilation [unit] is resolved.
    */
   final bool isResolved;
 
   /**
+   * The completion target.  This determines what part of the parse tree
+   * will receive the newly inserted text.
+   */
+  final CompletionTarget target;
+
+  /**
    * Initialize a newly created completion request based on the given arguments.
    */
   DartCompletionRequestImpl(
-      CompletionRequest request, this.unit, this.isResolved)
+      CompletionRequest request, this.unit, this.isResolved, this.target)
       : super(request.context, request.resourceProvider, request.source,
             request.offset);
 }