fix NSM error when completing imported inherited private fields

pkg/analysis_server/lib/src/services/completion/suggestion_builder.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.completion.suggestion.builder;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
@@ skipping 162 matching lines @@
    * Return the request on which the builder is operating.
    */
   DartCompletionRequest get request;
 
   /**
    * Add a suggestion based upon the given element.
    */
   void addSuggestion(Element element, {int relevance: DART_RELEVANCE_DEFAULT}) {
     if (element.isPrivate) {
       LibraryElement elementLibrary = element.library;
-      LibraryElement unitLibrary = request.unit.element.library;
+      CompilationUnitElement unitElem = request.unit.element;
+      if (unitElem == null) {
+        return;
+      }
+      LibraryElement unitLibrary = unitElem.library;
       if (elementLibrary != unitLibrary) {
         return;
       }
     }
     if (element.isSynthetic) {
       if (element is PropertyAccessorElement || element is FieldElement) {
         return;
       }
     }
     String completion = element.displayName;
@@ skipping 379 matching lines @@
    * or `false` if [computeFull] should be called.
    */
   bool computeFast(AstNode node);
 
   /**
    * Return a future that computes the suggestions given a fully resolved AST.
    * The future returns `true` if suggestions were added, else `false`.
    */
   Future<bool> computeFull(AstNode node);
 }