extract LabelContributor from local reference contributor

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.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.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
@@ skipping 101 matching lines @@
   @override
   Source librarySource;
 
   OpType _opType;
 
   @override
   CompletionTarget target;
 
   @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 ||
             _opType.includeVoidReturnSuggestions ||
             _opType.includeConstructorSuggestions);
   }
 
   @override
   LibraryElement get libraryElement {
     //TODO(danrubel) build the library element rather than all the declarations
@@ skipping 10 matching lines @@
   @override
   InterfaceType get objectType {
     if (_objectType == null) {
       Source coreUri = context.sourceFactory.forUri('dart:core');
       LibraryElement coreLib = context.getLibraryElement(coreUri);
       _objectType = coreLib.getType('Object').type;
     }
     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;
     if (librarySource == source) {
       libUnit = target.unit;
     } else if (librarySource != null) {
       // TODO(danrubel) only resolve the directives
       libUnit = await new AnalysisFutureHelper<CompilationUnit>(
               context,
               new LibrarySpecificUnit(librarySource, librarySource),
@@ skipping 58 matching lines @@
         dotTarget = node.realTarget;
       }
     }
     if (node is PrefixedIdentifier) {
       if (identical(node.identifier, target.entity)) {
         dotTarget = node.prefix;
       }
     }
   }
 }