Issue 22834. Disable 'Add type annotation' Quick Assist if the type is not accessible.

pkg/analysis_server/lib/src/services/correction/util.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.src.correction.util;
 
 import 'dart:math';
 
 import 'package:analysis_server/src/protocol.dart'
     show SourceChange, SourceEdit;
@@ skipping 983 matching lines @@
     Element element = type.element;
     if (element == null) {
       String source = type.toString();
       source = source.replaceAll('<dynamic>', '');
       source = source.replaceAll('<dynamic, dynamic>', '');
       return source;
     }
     // check if imported
     LibraryElement library = element.library;
     if (library != null && library != _library) {
+      // no source, if private
+      if (element.isPrivate) {
+        return null;
+      }
+      // ensure import
       ImportElement importElement = _getImportElement(element);
       if (importElement != null) {
         if (importElement.prefix != null) {
           sb.write(importElement.prefix.displayName);
           sb.write(".");
         }
       } else {
         librariesToImport.add(library);
       }
     }
@@ skipping 487 matching lines @@
 
   @override
   Object visitExpression(Expression node) {
     if (node is BinaryExpression && node.operator.type == groupOperatorType) {
       return super.visitNode(node);
     }
     operands.add(node);
     return null;
   }
 }