create new Dart specific completion contributor extension point

pkg/analysis_server/lib/src/services/completion/dart/inherited_contributor.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 services.completion.dart.invocation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart'
     show CompletionSuggestion, CompletionSuggestionKind, SourceChange;
 import 'package:analysis_server/src/protocol_server.dart' as protocol
     hide CompletionSuggestion, CompletionSuggestionKind;
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
-import 'package:analysis_server/src/provisional/completion/completion_dart.dart';
+import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_target.dart';
-import 'package:analysis_server/src/services/completion/dart_completion_manager.dart'
-    show DART_RELEVANCE_HIGH;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/source.dart';
-// import 'package:analysis_server/plugin/src/provisional/utilities/change_builder_dart.dart';
-// import 'package:analyzer/src/generated/engine.dart';
 
 /**
  * A completion contributor used to suggest replacing partial identifiers inside
  * a class declaration with templates for inherited members.
  */
 class InheritedContributor implements DartCompletionContributor {
   @override
   Future<List<CompletionSuggestion>> computeSuggestions(
       DartCompletionRequest request) async {
     // Determine if the target looks like a partial identifier
@@ skipping 132 matching lines @@
    * Return `true` if the given [classElement] directly declares a member with
    * the given [memberName].
    */
   bool _hasMember(ClassElement classElement, String memberName) {
     return classElement.getField(memberName) != null ||
         classElement.getGetter(memberName) != null ||
         classElement.getMethod(memberName) != null ||
         classElement.getSetter(memberName) != null;
   }
 }