extract InheritedReferenceContributor from imported reference contributor

pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart

Index: pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart
index f36dd7207c77052a5cd879235bf549a1bd613406..22cecaa8cfa8ad1120aed95e06db4f39824d139a 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.dart
@@ -11,6 +11,7 @@ import 'package:analysis_server/src/services/completion/dart/arglist_contributor
 import 'package:analysis_server/src/services/completion/dart/combinator_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/completion_manager.dart';
 import 'package:analysis_server/src/services/completion/dart/field_formal_contributor.dart';
+import 'package:analysis_server/src/services/completion/dart/inherited_reference_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/keyword_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/label_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/library_member_contributor.dart';
@@ -84,6 +85,8 @@ class DartCompletionPlugin implements Plugin {
     registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
         () => new FieldFormalContributor());
     registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
+        () => new InheritedReferenceContributor());
+    registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
         () => new KeywordContributor());
     registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
         () => new LabelContributor());