refactor ContributionSorter to use new API

pkg/analysis_server/lib/completion/completion_dart.dart

Index: pkg/analysis_server/lib/completion/completion_dart.dart
diff --git a/pkg/analysis_server/lib/completion/completion_dart.dart b/pkg/analysis_server/lib/completion/completion_dart.dart
index 669b2efe44192ef7ba223b5e6d366b463ab6eb7d..1ddd5a095be3d404a7928ebce2d61c7dc622d219 100644
--- a/pkg/analysis_server/lib/completion/completion_dart.dart
+++ b/pkg/analysis_server/lib/completion/completion_dart.dart
@@ -9,6 +9,7 @@ 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
@@ -78,7 +79,7 @@ abstract class DartCompletionRequest extends CompletionRequest {
    * 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.