refactor ContributionSorter to use new API

pkg/analysis_server/lib/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.completion.completion_dart;
 
 import 'package:analysis_server/completion/completion_core.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analysis_server/completion/dart/completion_target.dart';
 
 /**
  * An object used to produce completions for a specific error within a Dart
  * file. Completion contributors are long-lived objects and must not retain any
  * state between invocations of [computeSuggestions].
  *
  * Clients are expected to subtype this class when implementing plugins.
  */
 abstract class DartCompletionContributor extends CompletionContributor {
   @override
@@ skipping 49 matching lines @@
 
   /**
    * Cached information from a prior code completion operation.
    */
   //DartCompletionCache get cache;
 
   /**
    * Return the completion target.  This determines what part of the parse tree
    * will receive the newly inserted text.
    */
-  //CompletionTarget get target;
+  CompletionTarget get target;
 
   /**
    * Information about the types of suggestions that should be included.
    */
   //OpType get _optype;
 }