extract LabelContributor from local reference contributor

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart b/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
index a635b8cd76d4e0c0c120a3cfb1439cd32328476c..6b7e04fa37f2bd0f25a76060ce27f6c63246b0bf 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
@@ -119,9 +119,7 @@ class DartCompletionRequestImpl extends CompletionRequestImpl
 
   @override
   bool get includeIdentifiers {
-    if (_opType == null) {
-      _opType = new OpType.forCompletion(target, offset);
-    }
+    opType; // <<< ensure _opType is initialized
     return !_opType.isPrefixed &&
         (_opType.includeReturnValueSuggestions ||
             _opType.includeTypeNameSuggestions ||
@@ -152,6 +150,14 @@ class DartCompletionRequestImpl extends CompletionRequestImpl
     return _objectType;
   }
 
+  // For internal use only
+  OpType get opType {
+    if (_opType == null) {
+      _opType = new OpType.forCompletion(target, offset);
+    }
+    return _opType;
+  }
+
   @override
   Future<List<Directive>> resolveDirectives() async {
     CompilationUnit libUnit;