fix NSM error when completing imported inherited private fields

pkg/analysis_server/test/services/completion/imported_computer_test.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 test.services.completion.toplevel;
 
 import 'package:analysis_server/src/protocol.dart' as protocol
     show Element, ElementKind;
 import 'package:analysis_server/src/protocol.dart' hide Element, ElementKind;
 import 'package:analysis_server/src/services/completion/completion_manager.dart';
@@ skipping 203 matching lines @@
   }
 
   @override
   test_Block_inherited_imported() {
     return super.test_Block_inherited_imported().then((_) {
       assertCached('E');
       assertCached('F');
       assertNotCached('e1');
       assertNotCached('i2');
       assertNotCached('m1');
+      assertNotCached('_pf');
     });
   }
 
   test_Block_partial_results() {
     // Block  BlockFunctionBody  MethodDeclaration
     addSource('/testAB.dart', '''
       export "dart:math" hide max;
       class A {int x;}
       @deprecated D1() {int x;}
       class _B { }''');
@@ skipping 490 matching lines @@
   }
 
   @override
   test_partFile_TypeName2() {
     return super.test_partFile_TypeName2().then((_) {
       expect(request.cache.importKey,
           'library libA;import "/testB.dart";part "/testA.dart";');
     });
   }
 }