create new Dart specific completion contributor extension point

pkg/analysis_server/lib/src/services/completion/dart_completion_manager.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 services.completion.dart;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 12 matching lines @@
 import 'package:analysis_server/src/services/completion/local_reference_contributor.dart';
 import 'package:analysis_server/src/services/completion/optype.dart';
 import 'package:analysis_server/src/services/completion/prefixed_element_contributor.dart';
 import 'package:analysis_server/src/services/completion/uri_contributor.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart' hide AnalysisContextImpl;
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/source.dart';
 
-export 'package:analysis_server/src/provisional/completion/completion_dart.dart'
+export 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart'
     show
         DART_RELEVANCE_COMMON_USAGE,
         DART_RELEVANCE_DEFAULT,
         DART_RELEVANCE_HIGH,
         DART_RELEVANCE_INHERITED_ACCESSOR,
         DART_RELEVANCE_INHERITED_FIELD,
         DART_RELEVANCE_INHERITED_METHOD,
         DART_RELEVANCE_KEYWORD,
         DART_RELEVANCE_LOCAL_ACCESSOR,
         DART_RELEVANCE_LOCAL_FIELD,
@@ skipping 404 matching lines @@
             parameterNames: suggestion.parameterNames,
             parameterTypes: suggestion.parameterTypes,
             requiredParameterCount: suggestion.requiredParameterCount,
             hasNamedParameters: suggestion.hasNamedParameters,
             returnType: suggestion.returnType,
             element: suggestion.element);
       }
     }
   }
 }