refactor ContributionSorter to use new API

pkg/analysis_server/test/services/completion/completion_target_test.dart

 // Copyright (c) 2014, 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 test.services.completion.target;
 
-import 'package:analysis_server/src/services/completion/completion_target.dart';
+import 'package:analysis_server/completion/dart/completion_target.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../abstract_context.dart';
 import '../../utils.dart';
 
 main() {
   initializeTestEnvironment();
@@ skipping 532 matching lines @@
     addTestSource('main() {int b^ = 1;}');
     assertTarget('b = 1', 'int b = 1');
   }
 
   test_VariableDeclaration_lhs_identifier_before() {
     // VariableDeclaration  VariableDeclarationList
     addTestSource('main() {int ^b = 1;}');
     assertTarget('b = 1', 'int b = 1');
   }
 }