Handle null element in completion (issue 27461)

pkg/analysis_server/lib/src/services/completion/dart/suggestion_builder.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.suggestion.builder;
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analysis_server/src/protocol_server.dart'
     hide Element, ElementKind;
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
@@ skipping 11 matching lines @@
  * Return a suggestion based upon the given element
  * or `null` if a suggestion is not appropriate for the given element.
  * If the suggestion is not currently in scope, then specify
  * importForSource as the source to which an import should be added.
  */
 CompletionSuggestion createSuggestion(Element element,
     {String completion,
     CompletionSuggestionKind kind: CompletionSuggestionKind.INVOCATION,
     int relevance: DART_RELEVANCE_DEFAULT,
     Source importForSource}) {
+  if (element == null) {
+    return null;
+  }
   if (element is ExecutableElement && element.isOperator) {
     // Do not include operators in suggestions
     return null;
   }
   if (completion == null) {
     completion = element.displayName;
   }
   bool isDeprecated = element.isDeprecated;
   CompletionSuggestion suggestion = new CompletionSuggestion(
       kind,
@@ skipping 232 matching lines @@
   @override
   visitTopLevelVariableElement(TopLevelVariableElement element) {
     if (!typesOnly) {
       int relevance = element.library == containingLibrary
           ? DART_RELEVANCE_LOCAL_TOP_LEVEL_VARIABLE
           : DART_RELEVANCE_DEFAULT;
       addSuggestion(element, relevance: relevance);
     }
   }
 }